Also because Tier 3 campaigns are best as psuedo-sandboxes where you have a set of themed encounters a DM can snag to throw at the PCs depending on what the players decide to go.
I think only 4e pushed this and made creating mid level encounters dumb easy.
13th age has less levels, but that one also works to "high level", and with its mid level progression each level has 4 mini progressions so it can feel more like 4 levels.
And also PF2 has high level mechanics follow the same relative simple guidelines.
Bug sure its relative rare. And 5e just really does not look balanced enough to wanting to try tier 3. 5.5 may be better but original 5e was so bad that people wont really try, except for one shots.






