It has to do with Endwalker coming after the peak that was Shadowbringers. Shadowbringers was lightning in a bottle, and whatever follow-up they chose to do was likely never going to rise to equal it. At least in terms of presentation and story.
However, since Stormblood, the game has been on a downward trend regarding gameplay content, with the homogenization of jobs beginning then. Feedback from that first step in SHB objectively made every job simpler to play, particularly Tanks and Healers, and as such all gameplay related to combat (see: The Majority of the Gameplay) has suffered for it. Endwalker continued this trend, choosing flash over substance for nearly every job's rotations.
Then there's the fact that also since Stormblood, the amount of battle content has also gone down. Sure, they try new things and use that as an excuse for less, but we as players have no actual idea how much work goes into say a Eureka zone vs. a Dungeon zone. Or a Trial fight vs. a Dungeon zone. Or a sidequest chain with instanced battle vs. a dungeon zone.
FFXIV started in the valley of 1.0. A gorgeous game with high spec requirements and lovely worldbuilding. All dressed up to do nothing, ultimately though. Low point.
Then it rose up onto the hill of 2.0 with Yoshi-P's intervention, and sorta settled in to what it is gameplaywise, now, just with lower QoL. High Point, but later regarded as low point by new members to playerbase.
Then 3.0 came off the successful high point of 2.0, and delivered a juicy expansion in the form of Heavensward, which remains the only expansion to only add onto content in all areas without taking anything away or scaling anything back. High point, still regarded as the highest by veterans to the game, though some disagree.
Then 4.0 came, and it came riddled with issues from the beginning. They pruned job actions, killed Cross-Class skills in favor of Role skills, and only mitigated this by introducing two new jobs, both very simple in their execution (though the inner weeb of many wanted to believe Samurai was complex upon its introduction). Both jobs were also DPS. This expansion started giving us less dungeons every patch, only quipping in to say that the "innovation" of the Eureka zones was why. When the Eureka zones didn't even start showing up until 4.2 or 4.3 or whenever it was, and are essentially monster farms with fates (though I do personally like them). Raid quality was actually good, but Stormblood started the idea of checkpoints in fights with Exdeath > Neo Exdeath in its Savage raid series of Deltascape. In spite of being original with its antagonist in base 4.0, Zenos was received poorly by the playerbase at large, at the time, and I'd say is still. A lot of older players of XIV regard Stormblood as a low point at the time, but in later years look at it fondly, since the later expansions offer even less. We'll say low point, but it's still quite a bit better than 2.0 which was a high point in reality.
Shadowbringers burst on the scene at the start of summer 2019, and boy howdy, it was one hell of a ride that pretty much everyone enjoyed. However, battle classes became further simplified, so SHB is literally only carried by the strength of its narrative and its narrative presentation. Couple that with this being the point where there was an absolute deluge of new players to FFXIV who honestly don't know any better about the gameplay systems, and you get this highly lauded success where its downsides aren't perceptible by most of the people playing it. A high point for FFXIV, just for its sheer draw in of folks not already playing FFXIV.
Then we have Endwalker. Endwalker was supposed to be this epic conclusion where everything is epic and off the chain. It's supposed to be dramatic. It's supposed to show us the other side of the truth that we were shown in SHB. It's supposed to conclude Zenos at last. It's supposed to be glorious.
Yet... It was more or less mediocre. It failed to actually show truth, only willing to show us the other side of the truth we were told in SHB through eclectic metaphor. Its big world ending threat was barely a hiccup, and almost entirely unrelated to all prior expansions. There was no meaningful drama outside of the eclectic metaphor, and by the end of it, it really only wrapped up what was setup in StB/SHB on screen. All other narrative wrap ups are barely passing mentions. Towers? All went away nary a problem when we beat Anima. Final Days everywhere but Thavnair? Yeah, they kinda happened, but no biggie. Garlean leftover legions? Mostly disbanded, only maybe sorta kinda still out there somewhere.
EW's large positive reception is likely due to newbies again. They're still honeymooning with XIV, not longtime players. It's hard to hate on a game you only started playing 2 or 3 years ago, and it's hard to know better when you never experienced its previous iterations. Again it's not universal, but that's the crux of why some folks think of EW as a low point. I think it's a low point.
But, hey, if it's printing money for SE, they'll see it as a high point just like 2.0 was a high point, and we'll get more of the same, stale, hackneyed stuff we got in this "epic conclusion."