Why is it that people say you can’t gain muscle at planet fitness?

It's their opinion, and opinions are much like rectal orifices, everyone has one. Some opinions are also similar to rectal orifices, in that they are going to stink. I'd worry more about their contributions to global climate change with all of the hot air they expel, when they open their mouths to speak, than what their opinion is. If you want to build muscle at Planet Fitness, then get with the right trainer to guide you. But also find someone else who actually understands nutrition. A fact that many body builders doesn't really understand. 

Because they don't sell illegal performance enhancing supplements.

maybe cause they couldnt do it