Not female, but I know enough men like this to feel confident enough to answer. Also this really spiraled into a rant about the place of men near the end.
I think it's better to ask not why "Women like douche bags" and ask "Why do sociopaths all have such great hair?"
Seriously though, it's all about the type of person you're attracted to. Some women are just attracted to shitty men and sometimes confident, intelligent and assertive men are labeled as such without actually being terrible. They just share some of the same qualities. Hell, I used to be one of the guys who complained about it. Instead I worked on it. Complaining is easy. Working on it is hard.
Men who complain about why women want douche bags could well do to cultivate some of their mannerisms. Instead of arrogance, try out some confidence. Be assertive and honest about what you want, and think about if what you want is something rational and reasonable. Also hitting the gym and buying a flattering wardrobe couldn't hurt. If you have female friends that aren't attracted to you, tell them that you need an ally, that you want to work on yourself and get into the dating mix. Many women are fantastic for helping you in the dating scene. Instead the sort of man who whines about her dating douche bags will circle around that friendship with the female friend like a sort of pussy vulture, desperately trying to prey on her in a moment of emotional weakness.
So I realized that I was being a shithead. Surprisingly I realized it not exactly by being told, but by reading an article on why nice guys are anything but and I was almost all of these things that women hated. I stopped whining, I lost twenty pounds through hitting the gym and walking daily, I worked on my ability to talk and communicate with people (and developed a sense of humor), I stopped lying as a matter of course (more my own failing) and learned to either let people accept me or they could go hang. I became a better person and more desirable because I cultivated a better me from the dross that was the old me. My best friend who is getting married next month bemoans that she didn't meet me four years ago before her current fiance' (sadly not the healthiest of friendships, but a good friendship nonetheless) and I tell her that it would have been a waste of time. I would have driven her off with my shitty nice guy mannerisms. It'd be great if my current self met her younger self, but I guarantee that I would have tanked that relationship if we ever got into one.
However if a woman is actually attracted to shitty men then the person who asks that question is generally not the sort of man for her. This makes no sense. Why would she want to stay with a man who treats her like a servant, wants sex on tap, puts her down and maybe even abuses her? There is no rational reason because at no point in your life did you make a rational choice to be attracted to what you are attracted to. I highly doubt you made a decision to be attracted to men or women. I doubt that you made a decision if you're attracted to sporty men, or shy men, or abusive men. Long ago I accepted that rationality has very little to do with what I desire in life. Rationality only helps me nudge what I normally want in a better direction.
Some women are attracted to men who are bad for them just like the opposite, and that goes for same sex and trans couples as well. Some people are just attracted and/or attract crazy, fucked up people.
As for why TRP is growing and places like it, I feel that men largely feel like they are under attack. Some of this is due to the prestige and privilege of men being eroded by feminism. There used to be a man's place and a woman's place, and those gender roles were rather rigid for both. They still exist, but there has been a century and more of push back from women. The one good thing about gender roles is that at the very least it is something to conform to and it is socially accepted. Women knew their place and men knew their place too and either deviating from their places were often talked about negatively or shunned. Gender roles for women still exist, but it is also socially acceptable to challenge them to an extent with a minimum of judgement.
On the other hand, men often feel like they are socially adrift. A woman's "place" is whatever the hell she feels like now, or at least that is the attitude she can adopt if she wants to (or not). I on the other hand am constantly reminded by women and other men about my place as a man. I should be strong, a good provider and in a few cases sexually dominant and know what she wants in bed through what I assume is telepathy. I should make more money and woe be unto me if I were to choose to be a house dad, even if my wife/girlfriend were to make more money than me and it made sense to do so. There are also a myriad of things that I shouldn't be interested in because those things are feminine for some arbitrary reason. I actually do like some traditionally feminine things.
As a white man though without any inheritance and little help from family coming to me I am 99% on my own. I received no scholarships that I did not earn. I am not a minority and so my choice in colleges was diminished. If I were to have children my state has severe preferential treatment for custody going to the ex-wife. If I were to become homeless the amount of shelters that exist for me are limited. If you are at the fringes as a woman you have options. They may be absolutely shitty, but the options tend to exist. For men they don't. Your wife decides to throw things and beat you? There are no battered men's shelters and you are judged as a man for even being in that predicament in the first place. One is told to man up, which means to suck it up and take it.
I believe in equality of the sexes. Full stop. Men and women should be treated equally both socially and under the law. I don't consider myself a feminist, but I am 100% good with feminism as long as those practicing it don't try to step on me in the process.
Women have the dubious advantage of being historically oppressed and so they can make headway in legal systems and social ones as well. They can challenge those systems and shape their own way. Men instead have to cling to old values because there are few unified movements worth a damn that help address men's rights in a reasonable and rational way. When I hear "men's rights" I don't want to get involved, because it makes me think of movements like "white power". It's so filled with bigotry, misogyny and cultural baggage that addressing real issues is nearly impossible. I would love for there to be a real men's rights movement to address actual problems that men have without it being clogged with idiots. However until men get it together, get organized, dump those assholes from their ranks and spend decades changing the cultural narrative for men's rights to actually be taken seriously, it's not going to happen.
So anyway, what you asked was seemingly a simple question, but its answers are nuanced, complicated and wrapped in generations of social baggage. I doubt it'll even be detangled to my liking in my lifetime.