The minimum wage for waiters where I live in the US is $15.75/hour before tips. That’s the minimum, most decent waiters make significantly more. They tried to replace tipping with a 20% service charge here and it was the workers that rejected it, because they made less money than with tipping. Your model of how this world works is overly simplistic.

While variability may be stressful and I wouldn’t consider the lifestyle to be particularly healthy, many waiters consider it a feature associated with having flexible hours to do other things. If they want more hours, competent waiters can almost always pick up as many as they want and it has been a sellers market for waitstaff labor here for as long as I can remember.

Even illegal migrant workers made much more than minimum wage when I lived in rural flyover areas. A chronic labor shortage will do that.

To be clear, employers can't just assume that the tips bring them up to minimum wage

Yeah I'm tired of subsidizing someone else's workers.