Actually, your logic is flawed there. Certainly the Rams believe that Goff will be the far better QB long term. I would expect him to be better than Keenum at some point in the 2016 season even. That does NOT mean that he will automatically be better in game 1. He might be, but he might also need a little more time to get used to NFL speed, with more practice time coming to him than the typical #2, and possibly some playing time when the Rams are blowing out the other team, and then when he's basically ready a week of intensive practice - all the while he would be studying the book and game film as much as possible.
Keenum is already used to NFL game speed, and learning NFL systems, and while changes have been made, knows at least part of the new Rams' system. That may or may not mean he gives the Rams the better chance to win game 1.