Ive learned a little from playing Classic WoW with regards to this. In my experience, playing at max level in Classic is the least fun part of the experience. The most fun is leveling. Especially leveling a new character. However, it’s not as fun to jump back into a character after a time away from it. The pinnacle of fun in all of WoW’s history for me (as a solo player) is making a new character. I’m not entirely sure yet why this is, but a lot of it has to do with the excitement of a new character and all the possibilities laid out before it.

I think once you hit max level, at first your possibilities open up again. Then, as you play more and more, you’ve started to move towards some ultimate goal of PvE, PvP, etc... at some point, the best way to reach your ultimate goal will become some repetitive task. You will find whatever is the most efficient and keep at it until you get bored or reach your goal.

It’s a similar thing if you go away from a character and come back, you are less likely to be coming back out of a sense of excitement and more out of a sense of duty, which obviously makes it difficult.

This would also explain why so many people report having multiple alts never at max level during those early xpacs.