You should know that Erik ten Hag was successful in his first season at Manchester United, because Erik ten Hag managed to make Manchester United finish in third place in his first season. Therefore, even if, for example, Erik ten Hag was at Chelsea and was successful like that in his first season, then of course Chelsea would not sack him either.
Chelsea is not like Manchester United, the owners don't give second chance and they're not relenting when the appointed coach in charge looks incompetent. Todd Boehly sack Mauricio Pochettino and was bad news for the Blues because Mauricio Pochettino have sealed the europa spot but couldn't convert his chances. Manchester United hooked up with Erik Ten Hag because he was the man for the job. He deliver quite exceptional tasks for the growth of the Red Devils. Erik Ten Hag would have been successful in Chelsea.
But they will most likely keep Erik Ten Hag despite his poor performance this season. For the first season, it was a good thing that he could show because he was able to bring Manchester United to a top-flight finish, but this season he is in the middle of the table, and actually compared to Chelsea, both of them should have fired their coaches, even Erik Ten. Hag led Manchester United to win the FA Cup.
However, each team's management decisions are different, some make the decision to fire and some don't. This is completely their decision, because maybe on the other hand Manchester United wants to give Erik Ten Hag more opportunities. It's just whether Erik Ten Hag will answer with a good performance or not.
Although Manchester United won the FA Cup, Manchester United's overall performance was not good. I was not satisfied with Manchester United's performance. Even against lesser teams we have seen poor and erratic performances from Manchester United players.
Manchester United's performance in both attack and defense was poor. Chelsea scored 77 goals in 38 matches and Manchester United scored just 57. With such a weak attack it is impossible to be in the top 4 in the Premier League. Even with Ten Hag as coach, we don't seem to see any improvement in United's performance next season. Manchester United's performance is likely to be unstable in the next season as well.