The reason is hasn't been done is antiquated thinking.
Ok, blame it on everyone else's thinking, not on practical cost and reliability issues...
If you think GPS is dead, then there is no point in my spending further time on the subject. I am antiquated. (antiquated also got us to the moon in 69. Lets see your distributed systems do that.)
A manned system (to go to the moon) is going to be mainly centralized, gold plated and expensive. But a satellite system is not manned.