Why do airline stewards not ask for tips when staff in restaurants do?

I like cultures that discourage tipping. I think tips are just a sort of bribe; they smack of corruption (basically a bribe paid after the event rather than before) and should have no place in a modern society. The argument that wait-staff should expect tips can be applied to airline stewards, but tipping airline staff would be seen to be weird. Why should I tip a waiter but not an airline steward?

They are paid differently. I think airline stewards receive salaries while the waiters are paid a lower hourly wage.