It's obvious really they were doping. Chelsea simply struggled because they lost Costa, they still haven't recovered really. Leicester simply won the league because everybody else was useless, City were average under Pellegrini, it's hardly surprising that they fell away, Mancini fell out with too many players, just like Mourniho at Chelsea. United sold the best player in the world at the time and other players were coming to the end of their career, Arsenal suffered from the same issue.
There's an Iraqi guy on Twitter who quite rightly points out that up until 2018/2019 Liverpool were pretty average. This was when VVD, Robertson, TAA, Milner, Henderson, Salah, Mane, and Firmino were already at the club, it's not like they made great signings in the summer of 2018. There's no explanation how their form improved out of nowhere with the same players and they'd been playing with for 2/3 years.
Even in 2018/2019 they lost 3 of their group games on the way to the final and they were outplayed in those games. If Napoli hadn't missed 10 easy chances in Belgrade they would of gone through. If you look at the Chelsea side we lost to in the FA Cup final, their team was full of quality front to back, they should of got to the Champions League Final, outplayed arguably Barcelona's best side ever and they only finished 3rd that season.
Nobody can explain how average players at the RS transformed into world class players overnight. All the great sides recently, your great United, Chelsea, Arsenal, Man City, PSG, Juventus, Bayern Munich, Barcelona and Real Madrid sides have been full of players who were viewed as world class for a long time. While I hated him, Suarez was world class, he could score out of nowhere. Even when Eto played for us you could see he was a world class player and create something out of nothing. Every time he got the ball you worried, none of their front 3 have his skill and are anywhere near as good as players like Messi, Ronaldo, Mbappe, Neymar, Rooney, Bergkamp, Henry, Berbatov, Tevez, Drogba, Aguero or Dzeko.
Players like TAA and Robertson became world class overnight, yet now they're outplayed most games. Henderson and Milner were never viewed as great players, yet somehow gave more the the RS cause, that they ever side to other sides they were in, that were arguably better than their current side. There's nobody in their midfield that comes close to their previous midfield 3 of Gerrard, Alonso and Mascherano, they were undoubtedly world class, however they didn't win the league. All these players who dominated the Premier League have suddenly become useless and struggle and can't even beat West Brom at home. It just doesn't make sense.