Or he doesn't want to put too much pressure on a rookie QB and wants to give him time to develop without all the expectations building around him. In the end, it doesn't really matter who the #2 QB is. Carr is going to start as long as he is healthy and if he's hurt for any length of time, the season is pretty much so over. I think Rattler is going to be very good, but he still needs time to develop.
That being said, I could see a situation where Haener is brought in after a Carr injury if you have the lead and Rattler might get the nod if they have a week to prepare or you are behind.