You will be shocked by underestimating any team in the Champions league or in football generally. When Chelsea won Manchester City in the champions league final in 2021 nobody expected it. The book makers and pundits were backing Manchester City to win the final but it turned out that Chelsea won by a goal. Manchester City couldn't score a single goal for the entire 90 minutes. Football is not always one sided all the time, that's why it's difficult to predict it. Am not concluding that Bayern Munich can win Manchester City, real Madrid or inter Milan. What am saying is that Bayern is an elite team in the Champions league and they can win any of the teams listed above.
At least Chelsea at that time was one of strong team in Premier League, the previous final match was quite similar, Manchester City is a clear favorite and Inter isn't really a strong team. Inter can give a fight to Manchester City, but Manchester City is luckier and they won with one goal.
It's a mentality, sometimes miracle could happen just like Real Madrid that mostly won via comeback in the last season.
When we talk about the Champions League, mentality is something that must be discussed as well, because in the Champions League the pressure is very different from the domestic league. I don't mean to belittle the domestic league, especially the Premier League where we know what the competition is like. But in the Champions League it's different because it's the highest championship for a club in Europe, we can't deny that. There are even clubs that do make the Champions League their main target.
We are all just predicting here, we are predicting based on their performances during this season. When in the end our predictions miss or something surprising happens it becomes something very natural, because in football anything can happen. The bookmakers must also be thinking the same thing about a club's chances of winning the Champions League. But there are also other factors that we cannot simply eliminate in football, such as luck for example, or other external factors that make the course of the match not in accordance with what we predict.