As long as people play the content in numbers the devs consider "engaging" enough, they have no reason to stop, obviously 100 runs is good enough for most people so they keep farming it.

I also think people underestimate the amount of time they need to pour into this, if it's a 100 runs, and each run takes around 8-10m, this is around 15 hours of just doing an EX, if you get unlucky and need to do it for both that is around 30ish hours.
Than again if you play 4h a day and only do that and you never lose that is /only/ a bit more than a week to get both mounts...

Another issue I have with this is you cant even desync or GC trade the excess items you get from the EX, so you just farm with 0 profit unless you get a mount drop.