Because barely anything about this ending feels narratively satisfying.
The reality is:
Ash's story should have ended in a post-DP finale arc. Ash vs Tyson should have been top 4 in the Hoenn League. Ash should have accepted the Frontier Brain position. Timeskip (doesn't necessarily have to be an older Ash, but some indeterminate period of time where Ash is a FB and his skills improve). Then he goes to Sinnoh, where he should have won the league by beating Paul. Afterwards, there should have been a Champions League/PWC in a post-league arc like in AG, where Ash wins it all. He was ready by end of DP, both in temperament and skill.
Instead, the staff decided to drag Ash's character on for another 12+ years. Okay, whatever, he made a bunch more friends and caught more Pokemon. The BW 'reset' was horrible, but they rectified that in XY. And XY and SM were enjoyable series in their own unique ways. Ash did end up getting top 2 and Champion in both of those series, respectively.
The problem is Journeys. It didn't feel like it was supposed to be the final adventure of Ash, which it ended up. Ash had all these reserves, ready to be called up to help him win it all. Instead, he gets a BS fanservice team that he didn't need, and they themselves didn't get enough development. I don't hate Goh or Chloe, but look how easily former companions could have slotted in and out. Everyone could have gotten the remaining character development needed, and most of the reserves could have gotten a battle/focus episode/development. There still would have been ample opportunity for current generation promotion.
...So then MPM arrives. As I said in another thread, none of these episodes are boring - in fact, they're quite comfy and relaxing. However, they're in the wrong place at the wrong time. The reserves come back, but most of them are relegated to the background, and none of them got any focus besides Corphish (who lost a battle in criminal fashion). It's almost criminal that an iconic Pokemon like Ash's Charizard hasn't gotten a battle in almost 10 years. There's a bunch of dangling character and story beat left untouched, and Pokemon that could have benefitted from some needed screentime.
Moreover, the answer to the fabled question as to what makes a Pokemon Master is amounting to be, 'anyone who deeply connects with any Pokemon,' which Ash has been doing for most of the entire show. There's also a random Latias stalking him, but there's no deep overarching mini-arc with it. If anything, it should have been Ho-Oh, whom Ash captures in the end, thereby coming full circle. Meanwhile, Team Rocket is still doing the same.freaking.thing. They deserved at least a multi-episode finale arc with Giovanni.
Like I said, I love seeing the OG trio again, and the episodes themselves are not bad - some are even fairly good. I also don't have astronomic expectations for Anipoke, given the years of meh quality writing, constant merchandise/generation promotion in repetitive POTD/COTD episodes, and massive missed opportunities. However, I would have thought there should have been some no-brainer decisions for Journeys and MPM that anyone who passed Writing 101 would have done.
