Because 1) they would be able to trade with (or threaten) humans and 2) even ignoring that, humans behave differently towards obvious sentients—anti-slavery movements and anti-whale-oil movements are not comparable.
I think it is not that unlikely that they are roughly as biologically smart as us and have advanced forms of communication, but that they are just too alien and thus we haven’t deciphered them yet.
Why?
Because 1) they would be able to trade with (or threaten) humans and 2) even ignoring that, humans behave differently towards obvious sentients—anti-slavery movements and anti-whale-oil movements are not comparable.
I think it is not that unlikely that they are roughly as biologically smart as us and have advanced forms of communication, but that they are just too alien and thus we haven’t deciphered them yet.