If you believe that they still are, and if you happen to be right, it would be a legacy thing.
Otherwise, it's historical. Time was, no-one wanted to play one, and with good reason, for the most part (in terms of mechanics and play experience, generally) - so, they got bumped up, big time.
Class balance is such a funny thing, because it brings out such extreme claims and exclamations. Online, at least. . .
My experience of high level 3.5E play (let you know about high level pathfinder in about a year) is that full casters
could dominate above 15th level (only), but this is highly dependent on campaign style and assumptions. It's notable that a lot of the adventure paths top out around here:
Rise of the Runelords: 15th level
Curse of the Crimson Throne: 16th level
Second Darkness: 16th level
Legacy of Fire: 15th level
Council of Thieves: 13th level
Kingmaker: 15th level (but doesn't give a range)
Sepent's Skull: 15th level (but doesn't give a range)
The reason I bring this up is, if you accept that there is a caster/non-caster imbalance, most reports (and my own experience) suggest that it does not show up until you get to above these levels. My 3.5E experience involved combinations of feats, items and abilities that were perhaps not perfectly thought out -- none of these seem to exist in Pathfinder.
So it is worth considering that,
if the cleric is slightly too strong, it is an issue that won't show up (if at all) until the extreme end of the campaign. These also happen to be levels that the non-caster classes get some new and interesting abilities (look at weapon and armor mastery for the Fighter or the high level Paladin abilities).