People working in industries that get tips such as food services, delivery, hair dressing, and the like don't get salaries. Either they get paid hourly or commission. And because of that they aren't guaranteed a stable wage. Pushing for tips can be the way they cover for a shortfall in income when their employer shuffles their schedule towards less hours or a customer cancels an appointment.
As for raising wages... We've been trying to do that since 1993. There was a push in the '90s that wanted an increase from the $5 mark for federal minimum wage. There was an even larger movement called "Fifteen By Fifteen" which was designed around pushing the minimum wage up to $15 an hour by 2015 instead of the $7.25 it's been since 2009. Outside of the minimum wage, most hourly jobs in the U.S. have had their actual wages stagnate since 2011 when adjusted for inflation, if they've not outright decreased. Warehouse handling labour for example still only pays about $15 an hour national average, which when adjusted is actually less than it was in 2012 when it was $12 an hour. Working full time in that job without overtime ($15 an hour, 40 hour work weeks, 51 weeks a year) that's about $30,000 a year. For reference the average new car price nationwide is $48,000 a year (median $43,000), the average rent price for a two bedroom apartment is $1,900 a month as of May (median $1,600), and the average price of a house is $436,000 (median is the same although sources range from $420,000 to $480,000).
Non-specialist workers are kind of screwed over here in the States.
EDIT: Accidentally put rent as yearly instead of monthly.