Most things in life are achieved through hard work and effort.
Nice guys illogically mistake romantic relationships as working this way as well, when love and dating is actually completely and totally random! (and not in a good way).
They falsely believe you get out what you put in, you reap what you sow, etc.
They believe putting in the effort should see results.
They are simply frustrated when they don't see the results they were aiming for.
Some nice guys are manipulative ar55høles, but some are genuinely trying to be a 'nice guy' and trying their best but feel frustrated they aren't finding success when it seems plenty of men who are a55høles are.
I definitely believe this to be true.
Unfortunately, dating seems so random that effort means nothing.
You can improve yourself and this may help your chances slighly, but there are no guarantees.
There's plenty of kind, hardworking, social, friendly, confident, smart, fit, healthy people who are chronically single and never had a relationship, there's plenty of rude, selfish, greedy, unconfident/low-self-esteem, out-of-shape, uninteresting people who find successful relationship after successful relationship.
Nice guys identify with the label as they believe effort should bring reward, and so by associating themselves with a good person, they believe a good person is more likely than a bad person to find dating success, when this is not actually true at all whatsoever in any possible way at all, and never is, was or ever will be.
All of this lack of dating success and frustration with the realization their entire purpose to self-improve has been worthless and wrong, will make them feel a very strong Cognitive Dissonance. https://en.wikipedia.org/wiki/Cognitive_dissonance
So, consistently observing 'bad' people find much relationship success while they, a 'good' person remains alone, makes them angry that being a 'good' person doesn't work.
They start to think women do like a55høles for this very reason, which I believe has some truth to it, specifically a55høle women can like a55høle men.
Also, by constantly being told by society just how awful, boring, unattractive, rude, aggressive, manipulative, etc. they are, they eventually become the very thing they didn't like being called.
This happened to me in a similar way. I was a good kid bullied in 1st-3rd grade by both teachers and students, so all that built up hate and aggression meant I turned into the bully in 4th grade.
And it was only made worse and further ignited when people decide I was 'just an a55høle all along' when I only became this was slowly overtime.
This is what 'nice guys' tend to be.
Anyway, I am not a 'nice guy' myself, I'm an anti-hero at best and neutral, I've always identified as the exact middle of the 'good vs. evil' spectrum, but I may be mistaken as one even if I'm just being genuinely 'nice'.
I defend them simply because, to an extent, I can understand these strange creatures, but otherwise a lot of their train of thought has me confused, like why they'd naively think for a second effort in dating = reward in dating.
At best they are misguided everymen, at worst, possible creep who wants to drunk r*pe her in disguise. But that's a big gap.