Why do people believe this? The earth has limited resources
Eventually we'll hit that limit [1] but the idea that its going to happen soon is undefended. The issue right now for growth is that we can't mine the earth's resources fast enough right not that we've used it up.
[1]: https://en.wikipedia.org/wiki/Heat_death_of_the_universe
Efficiency of markets is like the Law of Large Numbers. But it's possible to get n consecutive coin flips that come up heads.