Because the losses are multiplicative, not additive.
If you have two things that are 80% efficient, you wind up with 64% of what you started with.
The sin (theta) losses and the reflective losses, which multiply together.
If it is impossible now, it is because there is not enough water.
There are LOTS of crops that just love the full heat of the desert if you can get enough water to them.
PV panels will not make water magically appear.