I imagine some of it is ego, some of it is jealousy, and some of it is ignorance.
Ego: Very few bronze players think they are bronze players. When they beat a higher ranked player, specifically Gold because that is likely their end goal in League, it confirms their held belief that they are better than their rank. If you make a mistake in lane or take a bad trade, the bronze player will likely all chat mock you because they are generally incapable of seeing their own mistakes, i.e. they believe they generally play perfectly. This is part of the reason they are bronze and prevents them from getting better at the game.
Jealousy: This one is fairly obvious. They think they deserve a better rank. However, all those other pesky bronze players keep holding them back! You must have gotten lucky to have climbed to gold.
Ignorance: Bronze players are there for a reason. Something regarding their own play is keeping them from climbing. I've met many, many bronze players who are pretty exceptional at their main champion, however they lack nearly all of the fundamentals of the game. To put it simply, they do not know what it takes to win a game. They win and lose games without truly knowing why. They make good plays without knowing why it's a good play, though more often they make bad plays for the same reason.
I realize you were just venting a bit but I thought I'd share my thoughts on a bigger league phenomenon in hopes that anyone who checks this out can discover something that's been holding them back from climbing. Cheers.