Is the answer slavery? Once you've been taken to Mars and given your underground living quarters, you're going to be stuck there and not have any option but to carry on working or be thrown out to die on the surface.
They should be doing that somewhere inhospitable to prove out the technology and concept. Set up in the arctic and work only with loads of materials that correspond to a starship load.
But then again no one is really serious about Mars.
Sure you can put people underground, but that’s probably not much fun. Why not just do that on earth?