It is quite difficult to adequately assess the chances of a team and put a normal odds when the opponent is a team that has never lost and is now entering its last match of the season. So I'm not surprised by these odds, although like many people I believed in Liverkusen's victory. They have come a long way anyway and left behind broken records, so they can be proud of themselves
Leverkusen's defeat at the end of the Europa League competition is something that should be said to be normal because Leverkusen has struggled quite extraordinary throughout this season in the Europa League. Moreover, Leverkusen's record in its own domestic league is also quite good this season so losing in the final match should not be considered a bad achievement for Leverkusen. So it is still quite natural for fans and Leverkusen itself to feel proud of what Leverkusen has achieved this season.
I agree with you that this is a normal result even though it occurred in a match that was not expected at all, but their defeat in this match does not in any way negate what they have done for a full season this season, they are extraordinary.
Winning and losing is something that definitely happens in football, we can't always want to see a team win or vice versa. And this season Leverkusen have tried their best to do their best, but what they want doesn't always produce good results, sometimes they have to experience defeat like this.
They must immediately forget about their defeat in the Europa League final, now they must focus on their preparations for next season. Remember their task next season will be tougher.