> Forget about why would they - they can't get here. The speed of light is too slow.

They could get here in a generation ship. Heck, we could almost build a generation ship that could reach another solar system.

First, there is the traditional generation ship where you set up a closed self sustaining ecosystem. I don't think we are anywhere near able to make such a closed system that would last for a trip to another star, but give it another few decades and there is probably a good chance we'll know how.

Second, there is another kind of generation ship that is called a seed ship or embryo ship. Your ship just carries a small crew, plus frozen eggs and sperm. During the trip as the crew ages you use artificial wombs to make babies from the frozen eggs and sperm and raise them to be the new crew. Any people you will need other than the crew at the destination (e.g., colonists if this is a colonizing mission) also come from the frozen eggs and sperm.

With enough automation the crew could be quite small. I once calculated the mass of high calorie density food that would be needed to sustain a small crew for thousands of years and it was actually small enough that I'd expect a civilization maybe a century more advanced in space than ours to maybe be able to manage. With that you don't need to solve the problem of making a closed self-sustaining ecosystem.

> With enough automation the crew [of a generation ship] could be quite small.

Other than the engineering challenges, we don't have the faintest idea how a really long-term crew is going to work socially and psychologically in a constrained, isolated, artificial environment.

The closest analog are the naval/polar expeditions, which were unisex and under military or hierarchical command, none of them longer than a couple of years. Some perished (Franklin), some pivoted (Shackleton w/Endurance), some failed with loss of life (Scott), some worked (Amundsen). But all had options unavailable to a ship in vacuum, and were on an incomparably shorter timescale.