Serious question, why don't NFL teams give backup quarterbacks real regular season game reps?

The situation the Packers, and other teams (Cardinals, Bucs) would not be so bad if the backup QB was given some game reps. Why is the NFL of the mentality that the starting QB has to be in there for every snap? Is there another position in any other sport that is similar? They won't even put in the backup to take the knee or hand off in obvious running out the clock situation at the end of the game...if he can't be trusted to do that you really should be looking elsewhere!