Because when you go to the eliminations outside of the group stage, every team can get surprising results. This year, the fight between Barcelona and Inter was the simplest example of this. While everyone expected Barcelona to win easily, Inter eliminated them. They made it to the final and played very badly against Paris Saint-Germain in the final. I don't know if Barcelona could have won the cup if they had made it to the final, but football is such a game, that's what I want to say.
That's to tell you that football is very unpredictable. Inter was able to eliminate Barcelona in the Champions League. Normally, Barcelona is considered to be a strong team, but it was surprising that Inter, who eliminated Barcelona, could not even challenge PSG in the game.
Barcelona was not just lucky, and had they made it to the finals, they would either have won the Champions League final or performed better than Inter.