In the 1990s that software was write-once, non-upgradeable, and bug-free because it was trivial. But it hasn't been that way for a long time. "Fuel injection requires software" turned into an exploit vector for feature creep.
The early electronic components were not reliable in an underhood environment and were not easily modified as engine control requirements advanced. Most of the 35 vehicles originally equipped with Electrojector were retrofitted with 4-barrel carburetors.
I'm guessing the rationale of the law is to prevent needing a repair company to exist & reinvent the electrical components of every single product in existence.