Why can't cars fly?

Answer 1

Flying requires lift, a force that counters gravity. Things that fly must use some sort propulsion (pushing large volumes of air) to directly counter gravity (like rockets) or (like propellers or turbines) to cause air movement across wings that provide lift. Controlled flight also requires controlling the direction of air flow.

Gliders are controlled falls, not really flight, because they have no propulsion.

Cars have propulsion only when the wheels are touching the ground. If they jump, gravity claims them immediately and they start to fall (accelleration toward ground first reduces speed away from ground, then increases speed toward ground).

Answer 2

they can . they are just called airplanes