re: Why do space suits need to be built with 2 legs and 2 arms?
Various interfaces similar to what you describe have been tried or thought about. The primary problem with any of them is a power failure or a mechanical failure strands the astronaut. I know you could argue but spacesuits could have a power failure or a mechanical failure and it would be fatal.
But they also don't allow humans to do what we do best.
We are pretty good ATVs and I'm willing to bet some spelunking will be required. Sure we get tired lugging around hundreds of pounds of stuff but a rover and a robotic cart with spare air would be better than robotic legs.