Their squad is weaker
From Mahrez to Doku LOL
From Gundogan to Nunes
From Laporte to Gvardiol (still adapting)
They also have/had the likes of Garcia, Bobb and Kalvin Phillips.
It's not that good anymore. In the past, they had top top options on their bench but that's not the case anymore. Their depth is weaker than it looks.
Is it that much weaker, though? If we compare their squad pre-Haaland to post, we see that:
In defence, they went from Laporte/Dias/Walker/Cancelo/Ake to adding Akanji/Gvardiol and having Stones become less injury prone. I'd call it a tie - but with a bigger upside (if Gvardiol works out) vs 2 years ago.
In midfield they went from Rodri/KDB/B Silva/Gundogan/Fernandino (at the end of his career) to the same first 3, but then adding Nunes (first season still), Kovacic and having Stones show he is a possibility at DM. Slightly weaker, with losing Gundogan - we will see how Nunes adapts over the long run.
In attack they went from Jesus/Sterling/Foden/Mahrez/Grealish (who never played that much - never got to 2000 mins in the league for instance despite never being injured much) to Haaland/Alvarez/Foden/Doku/Grealish. I'd argue that going from Jesus & Sterling to Haaland and Alvarez is a bigger "increase" in talent than going from Mahrez to Doku (as a 4th choice player).
So I'd argue that the main difference this season was losing KDB for significantly more minutes, and having their starters and backups being much younger (and therefore not near their peak) compared to 2-3 years ago. Doku/Nunes/Alvarez/Haaland/Gvardiol (who were their main buys the last 2 seasons) are 21/25/24/23/22 - compare that to Mahrez/Gundogan/Jesus/Sterling/Laporte who they replaced, who 2 years ago were 31/31/27/27/27 - all at their peak pretty much and experienced, and you can see it is that experience they are lacking more. That isn't to say they won't continue performing at this level - but just that young player are young players, and you usually don't expect them to perform as highly unless they are wonderkids.