What stops people from copying strategies?

Sounds like a stupid question, I know.

But let's say John joins a quant fund like Two Sigma (or RenTech or Citadel) and does well there. He then starts his own fund, copying 2Sig's entire setup, or at least the majority of their models and setup. John then raises money, makes returns, etc. Obviously he implements his own strategies and adds his own skill to the new fund, but by and large his great returns are based on his copying.

There are two cases: this is illegal or it isn't. If not, why doesn't everyone do it?

And if it is, how does any quant researcher or trader ever start their own fund? Wouldn't they necessarily be using strategies or models from their former employer to get started?

This is just a random thought I had. I don't know too much about this stuff so any help is appreciated. Thanks in advance.