The main reason we don't have desalination plants is that the water is more expensive than other sources, they're expensive to run, consume a lot of power, and you have to deal with the brine effluent.
Imported water and even purified recycled wastewater(potable reuse) are cheaper. In the case of the desalination plant in Carlsbad, desalinated water is about $2.1-2.4k per acre-foot whereas imported water is ~$1k and potable reuse had an estimate of $1.7-1.9k per acre-foot for the San Diego region(OC's potable reuse is cheaper at ~$800 per acre-foot, but trying to keep similar comparisons).
Power comprises most of the cost of desalination for the high pressures required for the membranes to work. I think it's 5 megawatt hours per acre-foot, but those were somewhat dated figures. While one could deal with blending the brine with wastewater treatment plant effluent or power plant cooling water, the greater reclamation of wastewater and the California Coastal Commission cracking down harder on the discharge of power plant cooling water is complicating things.
Desalination is an expensive commitment that produces at a slower rate than other sources. If you have a wet year, you're still stuck with the production commitment. It's great for water security though.
The main reason there aren't more desal plants is because ejecting the leftover brine into the ocean is an environmental concern.
That's exactly the problem.
There might be ways around it, though:
* Build 'em adjacent to (or as part of) existing "ugly" structures, like coastal power plants (which already use seawater for cooling/steam; I'd assume it should be possible to clean/desalinate the used water instead of letting it flow back into the ocean)
* Pipe the water further inland; the pipes should be easier to conceal and should require a smaller land purchase (at least in terms of meters of coastline), and could run to a desalination plant in cheaper, more-inland real estate
* Extend a well-deserved middle finger to those NIMBYs in the form of eminent domain. Of course, given that those same NIMBYs are the ones who have the means to bribe California's government every which way, that'll probably never happen... unless we the people make it a Proposition, of course, and force their hands by literally rewriting California's constitution to prioritize water for millions of people over ocean views for dozens.
The issue is that 99 percent of the time, you can get water a lot cheaper from another source. Due to the mechanics of reverse osmosis, desalination plants have to run all the time to keep the membranes from being damaged, so you end up paying for expensive water in the midst of the rainy season.
So no, nimbyism has nothing to do with it.