A bonus is not the same as a percentage cut of sales. Yes, bonuses exist and they can correlate with the success of a game in the best case, but they can (and also often) completely do not, plenty of stories of people getting laid off even if the game does well. These companies are so big they do not hold onto their staff as valuable assets, but as replaceable cogs. And it’s also why a lot of artists work on a contract basis and just don’t get any bonus to begin with.

And ‘to make up for extremely poor baseline salaries’… That’s not a thing as far as I know, and if it is where you are, it shouldn’t be a thing. It would be the game industry equivalent of tipping culture. Steal from workers ahead of time so you can punish them if the suits’ stupid business decisions don’t work out, awesome.

EDIT: Perhaps you’re referring to the fact that artists get paid badly at all, in which case, sure. But those bonuses aren’t to make up for that.