This may partly be because basically all of them are legal for highway use, so even if all you need is a car that is safe to drive a couple of miles to the grocery store at 35MPH, the safety features and engines (and thus emissions system capabilities) have to be designed with the expectation that the vehicle will be used at 80MPH beside all the other vehicles out there.
It might help to have a wider variety of vehicle licensing options to allow for low-speed, lower-spec ‘city cars’. Though enforcement would probably be an issue, and the safety standards would still need to be robust enough that people in small cars wouldn’t be too badly killed by the gigantic pickup trucks they have to share the road with.