It's because building anything at all is illegal. We have multiple canal systems built nearly a century ago that leak or can't run anywhere near capacity, because fixing them would (normatively) take two guys and a bulldozer but (descriptively) consume 10 years and billions of dollars of payoffs to a 100-turtle-deep layer of the usual suspects. We could easily desalinate another 10 million acre feet of water per year, but ... building anything at all is illegal.