I have heard this argument repeatedly from UFO sceptics. And the standard rebuttal of enthusiasts is that we don’t know everything about physics, worm holes, etc., etc. Which is true. And perhaps a civilization with thousands or millions of years of technological advancement beyond ours has overcome the physical challenges of traveling light years–as everything else, this is uncertain.
But another argument for the possibility of ET visitation, which we have not yet heard, is that of medical advancement. Is it not conceivable that an advanced civ might have developed medically or anatomically–essentially becoming immortal? Would a celestial trip of 40 or 50 years be anything more that a bucket drop? And even taking in the relativistic time effect of near light speed travel–would it be that big of a problem if your relatives and friends back on your home planet have aged by a few thousand years?