Understandable question. Sunk-cost fallacy. We paid a lot of money for the game, and we both thought it would eventually "click" or we would start having more fun (and we did honestly enjoy the campaign and leveling to around lvl 80). It just got duller to play, and the experience gained from doing a nightmare dungeon fell off a cliff. It also became useless to do helltides, world bosses, and legion events. PVP also seems non-existent (not that we were into that). Once we realized we were equally not enjoying it, we were in the high 80's, and it just made sense to 100% the season just to see something through to the end.

I have asked myself this, about so many people across all types of games, and even gaming as a whole. Unfortunately I can't offer any more insight than, that it is a very widespread idea and it definitely hurts a lot of people.

Disclaimer: I very much regret buying D4, which is also part of my FOMO issues.
But if you grew up with a beloved video games series it is hard to let go. I've done it in the past with other games. I basically boycott EA since ME3 (that was a hard one too, breaking away mid series) & Origin, and Ubisoft since Uplay. But it's hard when it is a franchise that you've spent hundreds or thousands of hours in, one that has basically no alternative like it. I know there's other hack'n slays, but nothing like Diablo's setting & atmosphere (maybe Wolcen, I don't own it, because apparently it is still a trashy mess).

And the worst part is that this is all happening to the whole market. It happened to mmorpgs before and killed it, and it happens to the rest of the industry now and kills it too. Something has to radically change, because this is definitely not fun anymore.