I suspect that many lush worlds would not have oxygen atmospheres.

Oxygen is really a deadly poison, to many theoretical lifeforms. There's a good chance that an alien landing on Earth would be unable to breathe our air.

Of course, it could be argued that we are well on our way to not being able to breathe Earth's air[0].

[0] https://www.youtube.com/watch?v=JPrAuF2f_oI