Question

for example:fire punch. could'nt that be a fire and fighting type move?

Comments

A grass water move on golem!

Answer 1

Another Reason is it would be really difficult and messy in calculation the defences . Fire punch to Ferrothorn will do like X6 damage .
Even i wondered about the same thing like a month ago but then realised how complicated the game would get .

Ex. Shadow Claw ( Chost + Fighting )
Does it hit normal pokemon , does it not !! Sooo complicated .

Answer 2

A fairly decent answer is the that it would add too much power to small attacks thus hindering the entire defensive spectrum of the game, a fire fighting move would indefinitely hit a steel type for a free 4x damage(or maybe more than that on Ferrothorn) almost guaranteeing a knockout every time, in other words there would be no defensive pokemon anymore, and even more typing to calculate in an attack would easily make the game less enjoyable. In other words, they pick one to keep it a simple as possible.