The short answer is Arch isn't actually good yet and Arch was supposedly advised by his uncles to sit behind Ewers for 2 years, learn from Sark, and start 3 years; so he isn't pressuring Sark to start him or threatening to transfer.
We saw Arch for 3 games against easy opponents early in the year. Arch already has a better deep ball and is more mobile, but he still seems to have some timing issues, he certainly didn't look like a clear upgrade.
Ewers has a lot more time in this offense and I bet he looks a lot better in practice. Ewers knows how to do everything and "can" execute at a high level, but he's just not close to consistent. As we saw on 4th and 13, he can make the big play in big moment. His main issue is the deep ball and usually he cracks like an egg under pressure. Ewers is a 3 year starter while Arch is a redshirt freshman. It's just hard to bench him for a guy who probably hasn't mastered the offense yet on the biggest stage.
As a fan I do wish he would bench Quinn because the offense is maddening to watch, but I'm not sure it would actually be smart. That's a lot of pressure on a freshman to just be inserted into the playoffs.