Futurology

Personally I think it’s probably just that our environment is rare.

That could be. Nobody has any idea under what conditions tectonics start; it could be super narrow. It may or may not be required, but if it is Earth might be exactly the right size and composition to allow long-term good conditions. Speaking of, there's also theories where planets migrate over time. If we had ended up too close to the sun we'd be another Venus.

This is all mitigated by just how many stars there are within nearby galaxies, though, so it has to be really unlikely for aliens to evolve and then roll out big things across interstellar space.

And then just let them deal with their challenges in order to mature.

That's implicitly assuming whig history, just saying.

And afaik evolution can settle like it did with the dinosaurs, where nothing much new happens.

Not quite, it didn't really settle. We compress like 200 million years of dinosaur evolution together in our imaginations, but that's not actually what happened. It's hard to say if technological bird-like creature would have happened without Chicxulub, but given what our ancestors looked like 70 million years ago and how smart existing birds are it doesn't seem far-fetched. Having worked with birds and heard about crocodilian behavior it would be a really weird civilisation, though. Maybe not even a viable one.

And further, why establish contact with humans?

Well, if they exist and expand, it wouldn't be a matter of calling us or not, they'd have to actively hide or we'd see them.