Because almost every empire has mapped only its tips and completely avoids the center of it.
Examples:
etc.
I get it, it's a desert, but still? They know it's there, they've encircled it and still "not ty"?
You answered your own question. It's a giant desert with no resources of value to any pre-modern society, and effectively controlling such a large landmass and all the peoples dwelling within would be worth exponentially more than would be gained. It was much cheaper to simply buy the loyalty of the peninsular natives and rule by proxy than incorporate it formerly into the empire.
