Why do you have to tip in America? Hi, I'm from India

i came from india i don't get it.Why American pay tips
i DON'T tip all of the following:
waiters, waitresses, food delivery people, gas station attendant if he cleans my window, the dry cleaners near holiday time, the newspaper boy, hotel concierge, and a host of others who may provide me with a service or accomodation
why would you tip someone for doing the work they are paid to do? If they have a problem with their salary, that is something they should sort out with their employer who is obviously robbing them. Customers should not be expected to make up any shortfall in wages.