I am all for interplanetary travel, assuming that survival on the destination planet is possible. The difficulty is getting there, of course.
I think that this is rather like time travel. If time travel will ever be invented, we might see them as they visit us. Since we have not any evidence of visitors from the future, it is likely that no one will ever invent a time machine.
The same may be true of interspace travel: it is possible, then we would get a look at them every so often. Being as it took them so long to get here, we could expect them to hang around. But so far this does not seem to have happened.
There was some nut who claimed that the earth was hollow, with holes at the poles, and aliens, possibly allied with Nazi survivors, lived inside the Earth. But I discredit this, though it is rather imaginitive.