There are too many evil outsiders around golarion, and there is almost no reason such creature havent totally consumed the planet except for plot convinience.
Ignoring the "plot convenience" and "it's a literary construct so that player groups will have lots of choice of prefab evil to fight" arguments for a moment...
All the source material neglects a glaring balancing factor of the disparity you mention -- namely, the number of adventurers in the world.
Assuming we have 250,000 Pathfinder Players in our universe, that equates to give or take Sixty Thousand 4- to 5- member adventuring groups, of widely varying power levels in Golarion, most of them unaware of one anothers' presence. Now, even accounting for not all of those groups active in the same parallel world-versions of Golarion, or travelling to and from other planes and planets, that's more than enough to make up for the Celestial Disparity you note. All those Pathfinder Society games? same world. And surely of every game group out there, there are a few on the same world, and just not aware of one another as noted above.
In summary, there's plenty of reasons why the disparity really doesn't exist -- if you don't want it to.
However, as popular tradition goes, we all enjoy stories where someone displays courage in the face of overwhelming odds and still wins, even at great cost.