For american cars at least, I read that one of the reasons this process exists is because car companies want to work around union rules for manufacturing by outsourcing components of the cars to subcontractors that they can make deals with.

Ultimately it's a price control strategy to pit these suppliers against each other to lower costs. But it means that designing these electronic sub-systems isn't just a question of the design itself, but also of managing all of these supplier relationships as well, they all have different contracts, you would have to coordinate all of them at once to make sure things are interoperable, etc.