Most septic tanks are not in coastal properties. The water table isn't going to change for a property unless you're right on the water or living on wide low lying march/delta type areas.
Check out this map of population density: https://vividmaps.com/wp-content/uploads/2021/10/metropolita...
All those dense spots in Florida and along the Gulf are also more likely than most US urban areas to use septic tanks. That's a lot of brackish poop, clogged toilets, and environmental contamination coming in the next few decades.
Miami-Dade is full of septic tank. I think the state has about ~3M of them total. No one thought about the coastal water table when installing them, and I honestly don't know if they do even now.
See: https://www.gainesville.com/story/sports/columns/dooley/2019...
Situation is similar across the whole Gulf Coast. You really shouldn't assume any of this was rationally planned.