You are on a right track here @pichlo but in your reasoning you forgot that environment changes over time and what may now look like an optimal and thrieving place, like the rainforest is only that because there are already countless types of organisms living in it, forming complex chains of interdependence.

Picture the same site 3 billion years ago when there was absolutely nothing else than bare rock under nitrogen sky bathed in unfiltered hard UV radiation... and suddenly a mineral-rich hot puddle of mud seems to look oddly attractive to life...