Why are physical comedies often described as "slapstick"?
Because actual "slapsticks" sticks used for slapping were used by the comedians of 16th century Italy.
From the definition of slapstick in Merriam-Webster's Unabridged dictionary:
a device consisting of two flat pieces of wood fastened together at one end but loose at the other and sometimes used by an actor in farce to make a loud noise in simulation of a severe blow
comedy that depends for its effect on fast, boisterous, and zany physical activity and horseplay (as the throwing of pies, the whacking of posteriors with a slapstick, chases, mugging)