Because that's
exactly what they are - you're not paying for the actual
teleporting lore wise, you're paying off the loan the citystates took out to rebuild and maintain the
aetheryte that acts as a beacon for teleporting to (without that, you would get lost in the aetherial sea with no way out). Of course the credibility of this lore is stretched a little in places like Zenith that supposedly have not had a mortal use them for
centuries and have no guards or attendents at all (and even then, why didn't
those aetherytes explode in the Calamity too?), but... suspension of disbelief is needed here.
