You get better at football by playing football. There isn't enough practice time to fully develop a QB if you start him and give him every practice rep, let alone with the massively limited work you can do if you if he's the backup.
Jordan Love looked like a rookie last year. Mahomes lost playoff games because he didn't know how to read a defense after wasting a year on the bench.
Either a guy learns from his experiences or he doesn't. But not playing doesn't give him a path to the learning that's required to play QB at a high level.