Learned behaviour. The easiest way for this perspective to be adopted is to witness game after game after game after game after game of people abusing the system and not being touched because they didn't use derogatory language while blatantly not trying to win via avoiding helping the team in any kind of fashion, stealing kills and basically just being on auto-pilot the entire game.
People eventually get sick and tired of feeling helpless when they have full rights to think that reports don't do anything substantial. They eventually cave and start doing the same. If they're smart about it, they can skate along the fine line of plausible deniability and be undetected for YEARS while knowing they are making everyone else miserable. If they are particularly bold enough to push the envelope and have no attachment to their account, they risk everything by poking the bear until the bubble bursts and either they get their VICTIMS reported or their own accounts are suspended/banned.
In the latter case, they simply start again because it's so easy to make a new account, rinse, and repeat.
I've lost a few accounts over the years as I am not a coward and call people out on their bull and take the 'eye for an eye' approach. But hey; I can't even remember the last time I played because I found a game which doesn't rely on a bunch of dips to do anything even remotely resembling teamwork and I'm actually having fun instead of essentially working a second job for no pay.
I'm tempted to actually update the client and play a few games now and then but I always ask myself: has the general rule of thumb of a PERSON being smart and PEOPLE being idiots changed any? The answer is always 'no, of course not!' so I just keep my stress down and relish in the comedic antics of various YouTubers playing the game and I feel that I'm pretty content with that.
My opinion is still subject to change as time goes on but at least I see things with clarity far more often now than I ever did while actively playing this poor excuse for a "game".
For those who can keep their cool and play honorably and let things slide like water off a duck's back...you have the patience of saints and I pray for you as those ulcers are slowly killing you.