The best teams in college football and the ones most deserving of a national title shot don't always overlap perfectly in a Venn diagram. If you go 13-0 with a bunch of close and unlikely wins, you might not be one of the best teams in the country, but you deserve a title chance. Consequently, if you beat 10 mostly good teams by mostly huge margins but slip up twice and don't win your conference title, you probably don't deserve a spot in a four-team field even if you're definitively awesome.
In 2019, the teams ranked first, third, fourth and sixth in the end-of-regular-season SP+ ratings reached the College Football Playoff.