But it is. The divines are bound to the mundus, and the thoughts of mortals are able to change them. Akatosh has other aspects because mortals think he does. The difference is, I think, in the numbers. In order for your ancestors to become actual gods, a LOT of people would have to believe it.
Akatosh exists, not solely because "Alessia said so", but because she said so so well, and the Marukhati did their dance to keep him around (in fact, if anything can TRULY be credited for Akatosh-as-friend-of-men, I'd hazard a guess that it WAS because they "broke him off" from Alduin, not just the "power of Imperial paradigm").
Again, with the exception of one source from Daggerfall, there are very few reasons to believe things are that flexible in TES, and even that had an entire "people" shaping the divine, not all the Backwoods Jim's wanting a Frog God to eat for dinner.
Edit: It should also be noted that the Daggerfall source also credits the "et'Adans" for the formation of the gods. Defining that with how we know et'Adans, it probably looked more like a kind of Convention (where, say, the world was planned out, and certain et'Ada chosen to give up portions of themselves to stabilize the Mundus, and the rest is history) than any kind of Ascension ritual.
Double Edit: I've done some more research on the topic. While I still feel that some over-estimate/over-stress the power of mythopoeia in the "common man's" life, it looks like I've also been underestimating it. Humble apologies for any misunderstandings.