
The playoff system rewards conference championship wins and is proven to not penalize championship losses, an example of this being last year where SMU lost the ACC but retained the spot they already earned. The championship game offers a unique opportunity: a chance to boost your seeding or earn a spot your record couldn't give you at the cost of playing an extra week of football (effectively losing a bye week).
The main folks who wouldn't want to play a conference championship are the top 4 seeds before championships, as they don't really need to climb the rankings any, but below that there's lots of motivation to compete with not much downside. Being on the bubble and winning gets you in, being 9-12 and winning gets you a home game, and being 5-8 and winning gets you a bye, though the AAC and ACC are competing for the bottom two seeds so it doesn't really apply to them, for them it's win or die.