Because most of the gamblers bet in favor of Real Madrid team, but the performance of this match today is directly opposite.
Madrid also performed badly in the home match against Barcelona. The Real Madrid team was defeated by a big margin of 4-0. Real Madrid's overall performance was shaky. However, we thought we would see Real Madrid's comeback in today's match. Because AC Milan's performance against opponents is erratic this season. So we all considered the Real Madrid team as the favorite.
Madrid tried to dominate their home match but failed. Madrid players attacked a lot in this match. Madrid players were able to make a total of 23 shots. 10 of these were on target. But even then Real Madrid players could not score more than one goal. The performances of Madrid's attacking players have been very erratic this season. Real Madrid strikers miss a lot of easy chances.
You are right that Real Madrid players performance is very uncertain and as time goes by there is a lack of confidence in them. Mbappe missed many scoring opportunities and as an experienced and valuable star player his performance was very disappointing. When Madrid's attacking players were busy scoring goals towards the end of the match AC Milan fooled the defense and scored more goals.