Laws vary widely from state to state, and some of the reason is local car club's involvement in the legal processes. It can vary from the difficult situation in Wisconson, which doesn't even have a bonded title processes, to Texas, which has a widely used one, to Kansas, where you only need a bill of sale for any car 1949 or older.
Missouri has provisions for "Hot rods", which is a vehicle that resembles a vehicle made before 1949, and "Classics", which is a vehicle resembling a vehicle made 1950 to 25 years old. doesn't matter when you put it together or if it is a new "kit car". It only has to meet the safety and emmissions of the year it resembles, not the year you built it.
How did this come about? Car club involvement in the political processes.
If you don't like the rules, organize, join a club and get active with the guys in the Capitol who have you by the balls