Why do flies survive being swatted ?

It will depend on how hard it was swatted and what cirstance. If a fly was swattted in mid air...it would travel from point A to point B in a stunned state and will try to revive by moving it's wings and legs. If it was swatted while on a wall it would killed outright. For a human it can be the same if someone is caught by a bomb blast and is projected through space,a person would be able to recover...but if blasted against a hard surface like a wall a person would die instantly.