Sure, there's probably some utopian nirvana regulation that gives you exactly what you need.
In practice airline regulation did preclude the airline from adding more seats. So in practice it banned airlines from offering you cheaper fares in return for enduring less legroom.
I don't pay a flat fee for my water, electricity, or gas usage, regardless of how much I use. I pay for the gallons, kWh, and therms I actually use. (Yes, there are other fees on those bills, but my usage actually matters.)
Airline regulation doesn't have to specify standardized seat pitch, etc.