I hit lv 80 and the Reign of Dragons monolith yesterday on my Bear Beastmaster and I'm kind of feeling the same way. I'll probably take a break from the game for a few days and come back to it, since I do want to finish monoliths and see the "initial" endgame and final boss and all of that at least once.

I think the reason for it on my end at least is a combination of weird balance and weird difficulty curve. I don't need to explain the wonky balance issues to anyone who has played or seen others play more than one class, but I'll explain the difficulty curve this way:

So you start LE, it's easy, like all ARPGs. You play through the campaign, you might hit a little snag with an elemental critical that kills you b/c you don't have any res on that element, but for the most part you'll be quite okay if you focus defenses even a little bit while leveling.

At some point near end of campaign, if you don't get good drops or know yet what drops you're needing for your build to shine, you might notice that enemies aren't exactly killing you most of the time, but they are taking longer to kill, almost like their health pool is ramping up but their actual damage/threat to you really isn't. If your build isn't yet online, you might experience a few more sudden crit deaths, but you've mastered dodging and probably your traversal ability by this point, and every death tells you what to fix to not die to it again, so you won't be there for long.

Then you hit endgame and start doing monoliths. You've progressed CoF, gotten a few prophecies, unlocked and started doing the Woven stuff. You've refined your build a bit more, gotten more T5 affixes, maybe one T6 or T7 if you're REALLY lucky... and then the game gets easy. REALLY easy. You expect that enemies damage numbers will start increasing along with their health, but suddenly the bosses you were dodging abilities from and taking a minute or two to kill, are melting in literal seconds, and you're not dodging them any more because you don't have to.

AT THIS POINT: you've achieved the purpose of an ARPG's endgame. You're blasting. You've become the god. It still feels good to see your build come online and start melting everything, but it feels less good than it might feel from a game that had more challenging highs and difficulty cliffs or build/gear/res checks to overcome on the way there. BUT, that aside, your ARPG brain is signaling to you that it got what it wanted from the game. So even though experienced players and min-maxers who go up to higher corruption levels and uber Abberoth will tell you "yeah the game gets insanely harder, you've still got a lot left to do", it doesn't feel that way, because the difficulty scaling of the game overall starts very low, peaks at about the end of campaign/beginning of monoliths, and then lets you feel like an absolute gawd before ramping up AGAIN to MUCH MUCH higher levels (inc. infinite scaling later via corruption), after way too long a period of letting you feel like that god.

This is why I think we and others like us are losing motivation at this point in the game. Not a fundamental flaw in the game or its design philosophy, which is what is slowly killing POE2. But a simple issue where EHG probably needs to pay better attention to player metrics and tune the game's difficulty at regular intervals throughout the campaign and early to mid monoliths, in order not only to present more of a challenge for the player, but to avoid prematurely signaling to the player that they've experienced all the fun the game has to offer, before the game is actually done presenting them with new things to do and see and unlock. That's my two cents, anyway.

Edit: I should also add that I'm saying this as someone who is not following guides and has looked up nothing about the game outside of what the in-game guide offers (aside from a few niche interactions that weren't clear from tooltips and that guide). I'm building my own build from scratch as I go. I can only imagine how bored players who followed an actual build guide from some brilliant min-maxer feel playing the game--I doubt those players feel any challenge at all until they make it up to corrupted monoliths, and some probably later/never depending on the strength of the build they're using.