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.