Some of the issue with automobile manufactures is that the vehicles are so complex and need so much capital that almost everybody who tries to build a new manufacturing company in this industry usually goes bankrupt. Tucker and DeLorean are really good examples of this, in spite of conspiracy theories that suggest ulterior motives of existing manufacturers.
The other issue is simply complying with government regulations in the industry. Some of those regulations certainly have been established because of major screw-ups in the past, but many of them (in spite of the manufacturers complaining about them) are enacted explicitly to discourage new entrants into the industry. At the very least the existing manufacturers only offer token resistances to things like seat belt and safety laws that add complexity as long as it hits everybody in the industry equally... and keeps new companies busy trying to catch up if they tried. If somebody built an exact replica of the Ford Model T, it couldn't be driven today except as a historical re-creation for off-road usage and certainly not something for mass production.