-snip- Although Manchester United spent a lot of money, they failed to select the right players for their squad.
Spending a lot of money will not make a team a strong team. A similar thing also happened to Chelsea in the previous few seasons where they made many mistakes in buying players. Chelsea has spent around 319 million pounds sterling to spend on players in the summer of 2023. And as a result, Chelsea only managed to finish in sixth place in the standings at the end of last season. They also failed to reach the UEL after United managed to win the FA Cup. I can say that Chelsea suffered a big loss because the results did not match expectations. So far only Cole Palmer has contributed significantly to the team and is Chelsea's best purchase so far.
-snip- Basically, big teams like Barcelona, Liverpool or Arsenal do not take Europa League seriously. This is why we don't see these big teams performing well in the Europa League.
I think the top teams also play seriously in UEL. The European League is the second prestigious league after the UCL so there is no reason for them to make the UEL just an amateur league. But there are several teams who are more concerned with chasing the domestic league champions and that makes them not prioritize UEL.