The manager. Specifically the setup. It's as simple as that.
Even with Pep or Klopp, it would take a miracle for this squad to be genuinely challenging. But the collective quality in the squad is far better than what we are seeing demonstrated on the pitch. We demonstrated this for large parts of last season.
It all comes down the the stubbornness of the manager. Last season, he came in and tried to implement a similar style of play for a couple of games, and we got spanked by two small clubs. He quickly realised the squad wasn't up to it, and reverted to a style we had been used to playing... a deep lying, compact 4231, which relied on counter attacks, where the main attacking threat came from wide areas where the "wingers" (especially Rashford) were able to play more narrow on the last line of defence, and the width was provided by the the FBs. I mean, Rashford has been awful this year, but the manager has to take a huge chunk of the blame for that. Playing to none of his strengths, and to all of his weaknesses.
Last years shape played to the strength of our midfielders too, as there was significantly less ground to cover defensively in the middle of the park. So the lack of legs was not exposed. Only City and Newcastle conceded less goals than us, and that is with a midfield 2 typically consisting of Casemiro and Eriksen. It also mitigated the lack of pace in our defence, as they clearly aren't suited to playing a high line.
But for whatever reason, without anywhere close to sufficient enough change in personnel, ETH has decided that the squad is now suited to play a completely different style. He wants to play a high press with his 5 attacking players, but with a solo DM, and a back 4 who simply aren't athletic or fast enough to play high enough up the pitch retain a compact shape.
So we get what we see every week. An absolute chasm between the front 5 and the back 6. So when opposition beat the initial press, a single opposing player can typically carry the ball unchallenged from one box to the other. It is just suicidal tactics.
And out of pure stubbornness, the manager will just not hold his hands up to the fact that he has got it all wrong, and go back to the way we were playing last year.