They started to, eventually. I think there's a 10 balls per second cap at the lower divisions of one of the bigger US leagues. But in most of the leagues I was in, there were no ramping modes - you'd just shoot as fast as you could, which was rarely more than 12bps.

Any rate of fire lower than that and it changes the game pretty dramatically.

They do, most tournaments have rate limits of 12-15 BPS now.