It is simple math. The nearest planetary system to us inilywsy galaxy is many light years away. It will require 200+ human lifespans to travel there at maximum speed our spacecraft can travel. That is the closest planets to earth
Others are much further away. Can you imagine producing 200 generations in a cramped space ship?
We are talking about aliens coming to Earth - not the other way around - so in that context your argument makes no sense.
And your math is wrong - A Centauri is 4 light years away.
Again the putative aliens could be eons ahead of us in science. Then there is James Clark Maxwell - who among the 196 discarded field equations proved it is possible to go from here to anywhere in the universe just by stepping there.
Maxwell, in case you are not aware, is the father of the electromagnetic spectrum, and the god-father of both Tesla and Einstein.
FTL travel is possible. Instantaneous travel is possible. The secret is that you must have ZERO MASS.
Many of the alien visitors are described as being ‘fuzzy’, ‘diaphanous’,’blurry’, etc.
One would assume, for instance, that ghosts have zero mass.
Maybe these ‘creatures’ have zero mass.