The weird thing is, we, as international guests were able to make advance FP+ reservations, which means that between 2013-2021, the international Disney World websites were able to make the reservations.
So, unless the website designers have forgotten how to do this, then you would think that it is to do with paying in advance.
However, I've just checked, and I can pay for a Cinderella's Royal Table reservation in full now, which means that I am able to pay in advance from my international country.
So I have previously been able to book in advance, and I have the ability to pay in advance, so why not allow us to book LLMP/LLSP in advance?
Even if we were unable to do it on the app until we arrived, allow us to do it on the website!