I’ll start by saying I don’t think that they are “so hated” anymore. There’s always going to be some people who don’t like them but in general there’s been a big move towards accepting them over the past few years. The overwhelming attitude used to be that they were cringey and irredeemable, as fandom has moved towards the “cringe culture is dead” idea that attitude fortunately isn’t as strong. People are a lot more live and let live about them now. There are plenty of fandoms on Tumblr and Ao3 where reader inserts/ocs are the predominant style of fanfiction.
I am a huge fan of reader insert fics and to a lesser extent oc fics. It’s most of what I read and write now. I love them, I think they’re incredibly fun… and I used to hate them. Before I’d ever actually read any I just assumed that they were all cringey, Wattpad level quality fics written by teenagers. I wasn’t mad that they existed, I just didn’t get them or have any interest in reading them. And then I fell in love with a character who had almost no canon fics, except for a ship that I did not enjoy. The other option was all of the character x reader fics about him…so I clicked on one of those, and four years later here we are. I discovered a whole new world of fanfiction that I’d never even considered before and I’m so glad that I did.
I assume a lot of people who don’t like them have the same opinion that I had and just assume the worst without giving them a chance. I’m sure some of those people might end up really liking them if they ever gave them a try. On the other hand, many people have tried them out and just can’t get into them which is totally fine. Some people are only interested in reading fics about canon characters or ships, some people can’t get past second person or the use of Y/N, and some people don’t want to insert themselves into a story (me neither, I mentally insert my OCs instead).
There are a lot of things to love about these stories, I think people would be surprised to find that many of them are beautifully written and contain really lovely prose and dialogue, the majority of them are in fact written by adults and not teenagers, and they open up a lot of fun possibilities to either imagine yourself or your own OCs playing around in these fictional worlds and interacting with your favorite characters. On the other hand there are legitimate reasons why that style of fanfiction just might not be a good fit for somebody.