Oh really? There are large water tables underlying eastern Nevada, the Sahara, and the Arabian Peninsula.
Much of it poison, unfortunately. There’s a reason many deserts are akali flats. And many of the deep aquifers are finite and have no source of replenishment.
When I was stationed at Holloman AFB in NM, CE put out a booklet when moving on base. It said, “The water on base is perfectly safe to drink, but do not use it to water plants.”
I lived in the desert for many years, and know how precious water is. Take a look at the impact of the California aqueducts if you want to see climate change in action.
In the US, many of the aquifers, like the Oglala, date back to the last ice age. Any surface water draining through all that rock would be marginal at best.