They're optimizing for time as knife cuts = time. A food processor will do it faster if you're more than one onion or so, assuming you can get the size you want.
Ahh, so in addition to having trouble getting consistently-sized pieces the size of a dice or chop, the other reason knives are preferred is that a food processor damages the onion, releasing more water compared to a knife. The result doesn't caramelize as well. This is why higher-end restaurants cut onions by hand, even when operating at scale.
You don't have to turn it into mush with a food processor, not all veggies are caramelized. High end restaurants usually optimize for speed but not over quality. Not sure why were talking that when this technique is for home chefs.
> The result doesn't caramelize as well.
Anecdotally I've prepared caramelized onions both ways, chopped with a knife and using a food processor and I've never noticed a difference. Onions have to release most of their water before they can begin caramelizing anyway so if anything, wouldn't that speed up the process?