Rain clouds are born in the ocean, not over the land.

Rain in California originates in the Pacific Ocean. Which is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the shops, but that's just peanuts to space the ocean.

Irrigation channels are tiny compared to the ocean. Those channels barely affect the local microclimate near the the channel; they don't significantly impacting soil moisture more than a few steps the the source and certainly not via evaporation.

The water loss to evaporation is an economic problem. California loses about 65MM gallons of irrigation water a year to evaporation. As a result, they need to build extra dams (or desalination plants) to produce an extra 63MM gallons of fresh water, just to compensate for water that never reaches the destination.

They could build big pipes to prevent losses, but those are expensive too.