Because there's a horrible disingenuous entitlement issue to the majority of people that label themselves as "nice guys". They bitterly resent women for not having sex with them and label them "sluts" for having normal sex lives that just happen to involve people that aren't them.
When did this become what people think nice people are ?
The whole nice guy vs. "nice guy" debacle has been a thing for way longer than it ever should be, I'm talking like the past 7 years, maybe 8. People keep asking the same question "why don't girls like me I'm a nice guy" and the answer is usually given, which is simple "if you're only being nice in order to goad the girl into having sex with you, then you're being dishonest on top of the fact that no matter how nice you are to someone, at no point are they obligated to give you sex for any reason whatsoever, no matter how long you've been consistently there for them or whatever you feel your accomplishments are".
But yet people keep asking why it's a thing. Like, it's nowhere near as deep as you think, it's pretty set in stone.
You really don't get what is going on.
I'm so tired of strawmaning "neckbeard nice guys"
It's not really strawmanning, it's just a thing that exists, man.
There's weird dudes on the internet who are vocal. ---
"Oh, my mother; oh, my friends, ask the angels, will I ever see heaven again?" - Laura Marling