Costs a lot of money. We can always make water given enough energy, and we have enough uranium to last us a few centuries if we want to build expensive nuclear plants. But no one wants to pay the cost.

When we say: there is a shortage of X, what we really mean is that there is a shortage of cheap X, and we aren't yet ready to pay the cost for expensive X.

Like almonds, they are yummy, but if the price went up by 10 because they had to use desalinated water, we might not eat so many.