(TLDR because I started rambling, but short version is that most restaurants are required by health codes to not make an effort to salvage or clean dropped food items because of how profoundly stupid a lot of cooks can be, which makes washing unreliable as a policy, even though it absolutely can be done effectively)

It's less to do with what's actually, logically safe and healthy and more to do with health code regulations, rather than just laziness. Yeah, if you drop something on the floor and it's practical to clean it, you absolutely can clean it and continue to use it without getting anyone sick. Unfortunately, for restaurants, you've got health authorities who can absolutely shut you down if they find out you don't do everything by the book (which washing and using dropped things isn't permitted in a lot of areas), so for restaurants it's almost always less of a headache to just throw it out if it drops, especially when you have a large number of employees, some of whom definitely aren't bright enough to remember to lie to a health inspector if they're ever asked about what's commonly done around the workplace (and who aren't bright enough to properly wash dropped things in the first place even if it were allowed)

Also, and more importantly, r/kitchenconfidential is a weird competitive circlejerk made up of professional cooks who are constantly trying to prove themselves as better than each other. So of course they'd never drop anything on the floor, and even if hypothetically they did, then of course they do everything perfectly and by the book, unlike all the other hacks there who are clearly just posing.