A car key having a RFID transponder in addition to correctly cut teeth to turn the ignition is a good additional security layer though.

If I were to guess, how we let cars turn into computers probably started with fuel injection. The same embedded system that controls the timing of injectors (in response to numerous sensors) and variable valve timing and whatnot.

From there, feature creep.

Next thing you know, you're getting an engine code due to forgetting to put the gas cap back on at the pump (paying money to reset that because it's still years before consumer OBD2 dongles become available) and it goes downhill from there.

:)