Possibly a similar reason we don't kill all the ants to take the sand?

> "Your argument #2 (that's the exact same as the zoo hypothesis) needs an extremely good reason for no alien at all trying to kill us and take Earth. And we currently lack any good reason."

Maybe life supporting planets are abundant and there's no reason to risk a conflict with us?

We've certainly been detecting a lot of them anyhow.

It could be equivalent to me going to Africa to fight a lion for its den instead of just renting an apartment.

It doesn't matter how plentiful planets are; and it doesn't matter if planets are not the aliens favorite environment. Living beings tend to get at every single place, unless something really compelling is stopping them.

(And we have a large history of killing lions to get their habitat. We do lot of that with ants too, and all kinds of insects like the sibling comment seems to ignore.)

If the Great Filter is behind us, I like mitochondria as a candidate; from what we know of Earth, it took over a billion years to happen, appears to have happened just once, and all complex life has it.

Two billion years, and it seems to be quite the fluke. One kind of single-celled organism ingested another, and instead of killing it or being killed, it provided a support system and in return got more energy back. This was a one-time thing that changed the course of evolution, making multi-cellular life possible.

Question is how often does that occur on other planets with life? Maybe Earth is unusual in that life sticks around for long enough that low probability events happen. Maybe it happened far earlier than it would normally take. The universe is big, low probability events will happen.

> This was a one-time thing that changed the course of evolution, making multi-cellular life possible.

Except that something similar happened at least twice. Plants have another cellular organelle that was created by ingesting bacteria.