That’s true, but intelligence is only necessary for certain kinds of success. Fruit flies, rats, cockroaches, and e. Coli are incredibly successful too. They can go pretty much everywhere and conquer the place in no time. They can’t really build stuff, but that’s not even needed for the kind of success they are aiming for.

Humans are social animals, just like rats and seagulls. They all need intelligence to understand their friends. This allows them to work together and succeed as a group.

Simpler animals may not have much intelligence, but they do have emotions. I would say that’s also a useful survival mechanism, but it’s just one tier below. When you have both, you open all sorts of interesting doors.