Because there's never been a country named "Palestine" and as such it's hard to make agreements with a country that doesn't exist.
The U.S. also recognizes passports issued by the PNA, though w/ caveats given the lack of nominal statehood.
Take Taiwan as another example. Both the context and details are completely different from that of the PNA, but it nonetheless contradicts your premise in the same manner.
Unlike Palestine.
The current State of Palestine exists, and is recognized in many forums, although not by all participants; but it doesn't have de facto sovereignty as it's essentially occupied.