We do that. They're called, "Reservoirs": Consider the Colorado River, where rain and snow runoff from mountains is diverted from that river and used for such things as agriculture. It's used so heavily, that the river does not run to the ocean any more and the resevoirs we've been using the river to fill up are at historical lows. So that's tapped out.

If you're thinking of using the runoff from local rains, consider that California is in a drought. Rainwater isn't going to be all that dependable. There are things called, "Aquifers" - the water table below ground. Rainwater is used to fill this up, and - well: those are also at historical lows because of wells.

Much of the rain that falls is actually evaporated up again, it doesn't run into the ocean.

So, we're still left with a problem of populations that don't have an adequate and cheap water supply. Uh oh.

Various entities in CA have spent a lot of money to prevent flooding, and in doing so they have made very effective systems that shunt freshwater right into the ocean. Because the water is very efficiently moved from streets to drains to cement rivers to whatever it doesn't get a chance to sink in, and that means that a lot of water is "wasted".

CA might be in a drought, but it still does get rainfall. Drought != no rain at all, it means less rain than usual.

If you don't get much rain, you have to do everything you can to prevent runoff. There's not much of that in CA and so there are problems. It's partially a drought, and partially bad water management that have caused the problems.