Fair enough. I had this discussion with my brother once who didn't see the point in D2 on Nightmare and Hell. I told him my opinion: that the game builds up to the higher difficulties, that beating the campaign on Hell mode (here, on Ultimate) is what the developers intended the player to set as the basic "challenge".
If you don't feel that way, then you just don't.
ARPGs are kill, get gear, kill, get gear. If the loop doesn't suck you in, if the gameplay itself doesn't keep you interested, than yeah, find something that does. You don't have any obligation to play further if you don't want.
If you bought tickets to a movie you didn't enjoy past the second act, you don't need to stay in your seat just because others told you it gets really good at the end.
Life is short, do what maximizes your enjoyment. That's why I eventually stopped playing EVE, Aion, FFXIV, POE...
Having said that, in my personal opinion, the game has barely started by the end of Normal/Veteran. You still have devotions to finish, a hundred+ skill points to invest, legendaries and sets to find, reputations to get up, Nemesis monsters to unlock, superbosses, SR and Crucible challenges, "roguelike"* chellenge dungeons...
If you want you could consider that "flesh" on top of the skeleton. Some people look at that content as the actual game, and the rest as prelude.
*The word roguelike is VERY loosely used here by the community. In this case it means you can't TP out of the dungeon and if you die, you have to use another key to start again.