I was under the impression that the mounts were something that was there to try to draw experienced players back to the extreme primals so that they would hopefully run with newer players and help keep players moving through content. Unfortunately, the drop rate is so low, combined with the 1/8 chance of actually winning the mount makes it infeasible to farm, so there is no reason for experienced players to go back and help with the extreme primals.
Even the mirror drop rate is too low to keep a decent number of people trying to farm it, which can be seen by how rare leviathan groups have become.
