I worked at BMW. I knew there was a project in there, using a certain ECU that was being quite problematic (as in, project being slightly late because ECU was a bit buggy and sometimes crashed when it was supposed to have almost 100% of uptime for legal reasons).

You ask: Why BMW doesn't just buy the ECU manufacturer?

Well... the company that was selling the ECU to BMW, is BIGGER than BMW. Even if BMW sold 100% of its assets and stock, it wouldn't have enough money to buy the ECU manufacturer.