My layman’s guess is that not all exoplanets have such fast orbits, but the ones with fast orbits are easier to identify and verify. Think about it this way: Neptune orbits around the sun once every 164 Earth-years. For an alien in a distant star system to spot Neptune, they would have to be in a star system that aligns with Neptune’s plane of orbit, *and* be mapping the Sun’s light output in that very brief window of time where Neptune obscures the sun. Even then, that wouldn’t be confirmation as from the alien’s perspective, it might’ve just been a dust cloud. They’d have to wait another 164 years to see it again in order to confirm that it was indeed an exoplanet.