Probably one of those sayings where accuracy yields to brevity.
I believe the saying originally applied to competing for women, where the "nice" guy typically also fails to project testosterone, rendering him less compelling to women, and so he "finishes last." You don't have to be a dick to be manly, but women nonetheless eat up the manliness and often give you a free pass on the dickishness. True story.
Perhaps unrelated, but around like, 26, I resolved to get in really, really good shape. I was just about to start my 2nd year of law school. I went hard at the gym 4 times a week or more every week, and ate pretty clean, not perfect, of course, but the junk basically left my diet.
So I come back for third year and the same hot-ish law school girls (and that type is there, because law school is, for many people, just a way to keep avoiding the real world) that never said a word to me during law school for two years are suddenly on my jock.
I could be a total dick to them and they'd be like "hahaha you're so funny!" or whatever other excuse they would make for my behavior. I'd literally say things like "those boots make you look kinda slutty, no?" and it would be a laugh or a comment like "yeah don't they?" I probably would have gotten arrested for that before I got jacked. You learn you can get away with a lot more if they're interested. It didn't matter that I was literally the same person they knew or knew of just a year or two before.