doctors and nurses have enough power to demand fixed professional(0) wages that "unskilled labor"(1) does not. no one _wants_ to make $2/hr(2) and to have to rely on the generosity of the general public for a living; in other words, it isn't the waitstaff having special privileges but rather the opposite case of them lacking better protections.
(0) which is to say, much higher (1) a propaganda term if there ever was one. work one shift as a waiter and tell me it take no skill afterwards! (2) $2.13 barring state-level increases over the federal minimum, to satisfy the pedants
so you tip garbage men? you tip macdonalds servers? you tip hospital cleaners? you tip schoolteachers?
I don't know how common it is anymore, but I vaguely remember people tipping their garbage men at Christmas.