I guess it’s a facet of the way they’re publishing it now. With annual storylines, I suppose the thinking is that they want people to play a campaign over a year and then move on to a new game. The rules for downtime are there to provide for stretching out in-game time so you don’t actually get to 20th level in 30-something days, but you don’t need to be playing for 5 actual years to do it.

I do agree that it seems fast when laid out like this though. One thing that interests me about Pathfinder is the idea of different XP tracks for differently paced games, although in practice most people just use the medium track anyway. Still, this is one reason why I mostly use milestone levelling in my games over awarding XP.