Because they don't have the imagination to come up with a "realistic" reason for using it. Because it's just there and not accessed through any sort of quest or other integration in the game world.
Because dragons, elves, and gods are realistic.
Here's a good reason
*Dragons have lust for power and knowledge
*Dragonborn has the soul of a dragon
*Dragonborn lusts for power and knowledge
*Dragonborn finds way to gain knowledge and power
*Dragonborn gets said knowledge and power
I mean, really. Is that not obvious to them?
Even if it's optional, they'll find a reason to complain, because they're so offended that it even exists, it doesn't matter if it's optional or not.
Like this rock here, in the road. I hate that rock. I don't have to walk over that rock, but still I hate that rock and I don't want it there.
That's the most perfect way to describe that.
Here is a cookie.
*Hands person the cookie*
Which gives me another idea. It's like being offered a cookie and getting offended because you don't like that kind of cookie even though you were never going to take it in the first place.