Question

air is mixture but not a compound why

Answer 1

compounds are mixed in equal proportion where as air us not mixed in equal proportion thus air is not a compound

Answer 2

A compound has to be chemically bonded, however, air is not chemically bonded.
This can be proven by freezing air. By freezing air, it yields different liquids at different temperature. Liquid nitrogen has a different boiling point than liquid oxygen.
If air was a compound, they would all have a single boiling point and a single freezing point.