I thought that the frequencies were originally decided by whatever machine the generator was driving. Want your big saw to split logs at 100RPM? Make your water

I thought that the frequencies were originally decided by whatever machine the generator was driving. Want your big saw to split logs at 100RPM? Make your water wheel drive a generator at 100Hz. IIRC every big motor was different and ran at its own frequency. Only when people started interconnecting these generators in a grid did the need to settle on a single frequency arrise.