Because all of the plants are fully amortized. If they were brand new, the total cost would be excessive. Customers are mostly just paying for operations, fuel and maintenance. There is no capital recovery needed.
margalabargalaOP
That's true of hydro heavy regions too, if not even more so. The Hoover Dam is not new. The Bonneville Dam is not new. Niagara Falls is not new.
https://en.m.wikipedia.org/wiki/Cost_of_electricity_by_sourc...