How would a space vessel be more hospitable than a planet with water, a partial atmosphere, caverns below solid rock, a small magnetosheath, and 62% of Earth's gravity?
*Edit: Instead of sending tonnes of metal into Earth's orbit from Earth, would it be more efficient to set up shop on Mars? Mine Mars and build space vessels in Martian orbit. Launching materials into Martian orbit should be relatively trivial compared with Earth.
The end goal of the generation ship would be to colonize a planet in a far flung solar system. I would imagine that with the number of planets in existence, we could find one that was more hospitable than mars. This would also provide a more robust 'backup' for our species, in the case that the extinction event was solar system wide.
Once again, I think Mars is a good stepping stone (for reasons that you mention), but I also imagine that a lot of the technology that would allow us to establish a sustainable colony on Mars would also allow us to develop robust generation ships.
*Edit: Instead of sending tonnes of metal into Earth's orbit from Earth, would it be more efficient to set up shop on Mars? Mine Mars and build space vessels in Martian orbit. Launching materials into Martian orbit should be relatively trivial compared with Earth.