If you make it to a final round and get rejected, why don't they consider you for other positions?

I'm talking about larger companies that have multiple very similar positions open. Let's say you make it to the final interview round, but then that hiring manager decides to go with another applicant who is very slightly better than you. You've been vetted, so shouldn't you skip the line and automatically go to a final round for another team at that company? At the very least, you're a much safer bet than some random applicant who hasnt even cleared the first technical interview yet. The only reason I can think that this wouldn't happen is if they discover some serious deficiency or red flag in you, but I'm guessing that that's not the case with most people who get rejected after a final round, most people got rejected because there was someone else who was just a little bit better (I'm making an assumption there so feel free to challenge that assumption if you think I'm wrong). The reason I ask is because I've made it to multiple final rounds in the past few months and got rejected, but most of these companies are large companies with many very similar positions. I'd be interested to know the thoughts of those with hiring experience.