If you build it yourself then some /many of the regulations don’t apply. For example you don’t need airbags (in Massachusetts). It varies by state:

https://www.semasan.com/resources/everything-you-need-regist...

Roughly the same thing is true for airplanes - experimental (“home built”) have to be airworthy but not certificated.

They say it’s due to long tradition of DIY vehicles in America.