Asisreo
Patron Badass
Playing High-Level D&D requires a much tighter understanding of lore, settings, spells, and game structure. At low levels, you can say "Hey, there was an ancient King that traveled to the northern mountains of Loridyne and he was imprisoned for all eternity for his greed," and the players kinda just now know a chunk of history.Why would you get worn out? And if you're worn out playing D&D, how is playing yet more D&D at low levels any less wearing?
At high levels, that king isn't just someone they can visit, its possible that they have the resources and abilities to un-imprison, resurrect, and speak with this ancient king and you, as the DM, have to figure out what he would say and exactly what he knows.
Lower levels have inherently more restrictions for the players but high levels remove many of those restrictions and a DM must take account of that.