The College Football Playoff is an invitational. Even the week to week rankings are arbitrary and decided by a group of people who will not share their reasons. 

The selection committee always leaves someone out who many people think should have been in. And likewise they put someone in that many people think should be left out. 

Until the committee is eliminated entirely and there is a defined 'play in' system, we will always get this debate of who 'deserved' to be there.