Because it's not how GPS works.
Due to relativistic effects, "position of the satellites" is a meaningless concept in this context.
It would be a useless bit of "information" to transmit; one might as well transmit old episodes of Bleep and Booster.
Some of the relativistic corrections required to make the system accurate enough to be worth using can be made on the SV, but
others can only be performed by the receiver, because the relative movements of both the observed (SV) and the observer (receiver) must be taken into account.
(
"relative movements", "relativity", geddit? 
).