I don't know what happened to MU, they haven't been able to get out of the inconsistent zone. The team development carried out by several managers after Sir Alex Ferguson, could not bring glory to that era. It's difficult to find the right replacement of Sir Alex Ferguson. Arsenal is lucky after Arsen Wenger, currently there is Arteta who seems to be able to make Arsenal grow even though he experienced a difficult phase which made Arteta receive a lot of criticism from fans.
Manchester United glory have been over since leaving by Sir Alex Ferguson and several manager failed for winning domestic league until champion league trophies. Have been 12 years ago since first time David Moyes until right now Erik ten Hag become the manager still not any one of them success make Manchester United finish on the top standings position or becoming the competitor for Manchester City for winning Premier League trophy.
Difference with Arsenal, they had success with Mikel Arteta become the manager for replacing Arsene Wenger position, indeed still not winning any trophy yet but Arsenal keep solid performance and always finish on the top fourth standings exactly last season almost close become the winner of premier league.
The last time Man United won the Premier League Trophy was in the 2012-2013 season, so it has been 11 years since they could not get the trophy again. I think Man United no longer has that winning mentality and that's the reason why Man United is so bad at the moment. It's not just because they haven't found the right coach yet, but they also have lost the winning spirit they once had. With their current condition, it is clearly not easy to return to glory. Maybe apart from coaches, Man United also needs to improve their academy development again and give more opportunities for their academy's players to contribute in real matches.