There is nothing that would guarantee that FP+ are not going to run out. If they did it each day, the first X number of guests would still get FPs, and the rest would be out of luck. Also, imagine the annoyance of having to log into the system each day for 10 days of your trip and have to schedule your FPs.
If the system works well at all is up to personal tastes, but as-implemented, being able to book for your entire trip is an advantage over having to do it each day individually.
And to your second point, at least for off-site guests, that's a design choice as well. They want to entice people to stay in their (overpriced) resorts on-site. Having access to FP+ earlier than others is one potential perk to convince someone to stay onsite.
Have not honestly followed AP holders access though, as I don't see a time in QUITE a few years that I'll hold an AP, so I have not thought about their access much, being 100% honest.
