According to Todd Howard, here's why there's no land vehicles in Starfield

Here’s the totally coherent explanation from Todd:
“It’s something we considered. We’re gonna do outer-space, we’re gonna do planets. Once you do vehicles, it does change the game-play,” said Howard. “When you’re on foot it lets us really, for the players, make it an experience where we know how fast they’re seeing things. In one sense, you do have a vehicle – obviously you have your space ship.” He continues “but then on the surface, you do have a jetpack … and obviously planets have different levels of gravity which make that unique.”