Fair point, PmP.

But there is a shortage of water both on Mars and in the Sahara desert. In the Martian gravity you may be able to do with 90% less water than on Earth, but that is not helpful if you have no water at all, or if it costs you $100,000 a kilo (I am guessing here) to ferry it from Earth.

The Sahara water shortage, while not intractable, might turn out to be an affordable project if food crops flourish in an irrigated desert. On the minus side, since the Sahara surface cover consists mainly of sand over a large proportion of its area, it scarcely appeals as a place to find fertile soil.