I think the real obstacle isn't even the regulations but the safety systems. The various US DMVs can comprehend things like scratch-built or kit cars, but the level of engineering to make a thing that can even sometimes decelerate a person from like 60 to 0 without killing them more with exploding airbags is several levels above that required to make a thing with wheels that drives forward.
So you can build and probably even drive a car from plans you got off Github, but if you crash it it will kill you.