Yea, some are definitely made by amateurs, but there are many that don't need crazy hardware, too.

The reason the big budget games look good is because full time artists and full time optimization of assets is a whole-ass job many indie teams won't have the budget for. Creating art is one thing. Making it render efficiently is a whole other ball game.

Game engines are getting better at helping small teams, though. Unreal Engine has quite a large suite of tools just for cleaning up and generating lower detail models and baking in vertex/parallax mapping, adding culling planes to maps so it doesn't render the whole thing at once, etc. It is a HUGE task to take good looking assets and also make them render quickly. Especially if they avoid something with tons of provided tools like UE. Then they have to do all that optimization in other ways or just skip it.