I have numbness in my butt and feet when I sit for too long, why?

Pressure neuropathy. Any time one is putting pressure in an area where nerves are located in places easily reached by persistent pressure, will cause temporary damage to those nerves (like sleeping in the wrong position, holding arms over head too long, sitting too long--esp hard surface). When the pressure is relieved, the nerve function comes back and numbness goes away. The longer the abnormal pressure, the worse.