Personal preference, and based a lot on how people approach cornering. I run 810mm bars on my trail bike and 780mm on my XC hardtail [50mm and 70mm stems respectively] (6'2", +3 ape index - somewhat broad shoulders, so I'm not totally off the deep end with this). I try to attack turns on initiation, and want stability when I'm mindlessly plowing through rocky sections on both bikes, so I run wider bars. I'm not a finesse rider, and manage to collide with trailside trees and cacti even running 680mm bars, so I lose comparatively little.
I still use as my primary metric for bar width being adequate how much push-up force I can generate at halfway through pushup position - which is where my poorly built upper body is basically at its weakest. I think it biases me towards wider bars, but my compensation mechanism has been to go full 'duro and slap on some handguards for the cacti and drive on.
I think a non-trivial amount of the hype for wider bars is that it's always possible to trim bars down slightly - for each person it's individual preference. I think the reason the default answer is always 'buy wider bars' is that cutting down bars to fine tune is always possible, and trying wider bars with shorter stems typically comes with more enjoyment, and among the few who decide to trim down, it's usually not all the way back to the original size.