The low operating temperature from a combination of fans blowing air everywhere, and not generally running at ratings at all times (most people aren't f@h, cryptomining, etc.), and also either intermittent use (shorter "engine hours" so to speak), or maybe reduced temp cycles from always-on operation, helps out.
AFAIK, polymers are just as prone to failure as electrolytics, they just tend not to run as hot. Both can last quite a long time when not abused. And the luck of the draw, and the confirmation bias where you forget about the ones you toss out, while the ones that stick around are there to actively remind you.
From personal experience, I forget what earlier mobos/PSUs died of; one mobo, the caps split open, that was pretty obvious. Forget what PSUs were, if it was fan death leading to smoking electronics, or caps again. Think the last one or two were at least fan related. My current PSU seems to be pretty reliable besides the fan, which I'd already caught twice running out of lubrication and making noise and stalling; finally, earlier this year, I put in a (brand name, ball bearing) replacement which should outlast everything else pretty significantly.
Tim