I don’t know, but if I had to guess like everything else it comes down to money. It’s energy intensive to desalinate seawater to the degree it’s drinkable, and now we’re talking about adding even more energy to refine it even further to make it suitable for human consumption. That makes any recovered salt expensive compared to natural salt deposits. Much easier (read: cheaper) to just scrape salt deposits that have already evaporated.
Trump exhibits zero-sum thinking- if someone else is getting something, that means he is losing something. Thus if Carter is getting respect, Trump thinks he is losing respect.