It's the same reason Florida St should have been in the playoff last year. Did anyone think they were really a better team than Alabama? No. But they'd earned it on the field.
If you put BYU up against Alabama, I'm sure Alabama would be favored, most would say Alabama is 'better'... but Alabama has also lost games and BYU has won. Even if they're games that we "shouldn't" have won or games where we had some luck to get the outcome.
Realistically, if we played the last two minutes of that Utah game 100 times, we probably lose more often than we win. But the only outcome that should actually matter is what actually happened. And what actually happened is that we won.
If you're looking at team quality, then hypotheticals matter...but if you're looking at what team deserves to be in (which is what rankings should be), then we're clearly ahead of those teams