Ghanima_Atreides wrote: »Apart from the fact that it would be lore-breaking for many Dragons to be around at this point in time, Dragons aren't animals to be trained and mounted at will. They are extremely proud, intelligent beings who consider themselves above mortals (in fact, they once ruled over mortals as gods). In Skyrim, one accepted it - once - under duress, and the Dragonborn could force others into it by literally bending their will with a special Shout that only a Dragonborn could use. So no, no Dragon mounts in ESO, ever.

Burninating the countrysideOldmanlawlor wrote: »A dagron like Trogdor?!
I want one if Zos would put it in it would cost like 3,000 crowns at least
Put simply, because it'd be pretty lore-breaking to have them flying around everywhere.
Skyrim was set about 1,000 years after this game. You'll know from Skyrim that many of the dragons you fought were resurrected from death by Alduin. At this point in time, most of them are still dead from the "Dragon War" which took place before ESO. There are dragons who are alive in this time period, yes, but they're pretty deep in hiding. And they're certainty not being ridden around!
Edit: Here's a more detailed explanation: uesp.net/wiki/Lore:Dragons