No, it's because vehicles before then were not required to meet minimum sound requirements by law. Check the spec sheets of HEV/PHEV/EV vehicles made before then -- they don't have acoustic alerting systems. Now they all do (in the jurisdictions that require it).

The law didn’t exist until there were enough electric cars to care about this. The cause and effect starts with the number of quiet cars.

Yes, but it wasn’t their mere existence that caused automakers to add noise. People who buy these vehicles like that they are quiet. The noise generators were added only because regulatory action was taken.