Because a situation like is pictured in the OP's photo is exceedingly rare. On the whole, she appears to be very attractive and he appears to be very unattractive.

A female of her attractiveness would have no trouble attracting many alpha/rich/popular/funny/whatever guys who are also easy on the eyes. Women don't generally just choose a funny guy and go for it, they choose a funny and rich guy or a funny and attractive guy - they are also usually know roughly how attractive they are and almost always seek out someone equally or more attractive - unless they are a gold digger. There is also a bit of a misconception on reddit about "gold diggers" being any women who is looking for a guy that makes good money. Most women DO look for this in the same way they look for a guy who is smart/funny/rich - they usually say things like "looking for stability". I don't consider this gold digging, I would say gold digging is looking for money above all else, or looking specifically for guys with LOTS of money (multiple millions) in which case they can't really care about looks at all.

And the whole found someone, fell in love and happily ever after scenario. Many women her age (20's) would dump their partner rather quickly if he became that unattractive and simply start dating one of the many guys who hit on her every day.

So with all that said, this picture really irks many of the men of reddit.
For the past 20ish years, society has been on a "you're special" kick. Most of the people on reddit are younger men and were brought up to believe they're special - without really defining what makes them special. At the same time, society, and especially media, has taught them they deserve, and will get a girl. And not just any girl, but an attractive girl.
But now as they grow up, they are realizing that this was a lie. Girls don't just fall into their lap. Especially not attractive ones - unless they themselves are attractive as well (the whole be attractive, don't be unattractive thing).
So, guys see the OP's picture and get angry that someone who is less attractive than they are, got the hot girl that they deserve.

tldr; attractive women are generally very selective in choosing a partner, men are generally upset because they don't have a partner (or don't have the attractive partner that they deserve), but this unattractive guy apparently does.