Because the real problem is inland, so as well as desalinating the water (which is somewhat easier said than done, though I agree in principle), it would have to be pumped inland and mostly uphill for millions of miles.Our state water infrastructure is ancient, we just last year secured passage of a $9m bond to overhaul it, despite the vociferous objections of the farm lobby.
While a pipeline network may not be millions of miles from one end to another, it's certainly possible that the sum of the lengths of all the pipes in a tree-like network may be much greater than the length of the network's longest path.
If they build desalinization plants wouldn't they just pipe the water into existing infrastructure? At worst that would mean piping it over to the other side of the state where they usually get their water from snow pack and what not, no need to completely duplicate their water distribution system just because they have a new source of water.
Desal plants are at sea level on the coast. Most headwaters are thousands of feet above sea level and hundreds of miles inland.
Water infrastructure is almost entirely gravity fed. Even if we could justify the immense energy usage, we can't drive existing pipes and canals in reverse.
