This is true, but remember that averages can be misleading. Available water is a very spiky graph [1]. Reservoirs balance it out some, but cities need and can afford reliable water, so backup sources are good, even if they're expensive.

[1] https://d32ogoqmya1dw8.cloudfront.net/images/trex/students/l...

From: https://serc.carleton.edu/trex/students/labs/lab4_1.html