Actually they do have a blog entry on it:
In 2009, PayPal rolled out a set of very similar API's that would allow them to do these things (see http://x.com). I would guess that the fact that they haven't made the switch is just a matter of inertia, the difficulty the transition might present, and having more important things to work on, but I'm not intimately familiar enough to know if there are other reasons they might be staying.
I do know that a friend of mine had a startup that started on FPS but was very, very eager to switch to PayPal when X came along because FPS presented so many problems. They were a lot smaller and less wedded to the platform, though.
Disclaimer: it's my brother's company