I knew this would be too good to be true.
Taking CoD to the far future is a risky move for the fanbase, so creating a remake of the most renowned title in the franchise will be a great incentive if bundled with the game. It will undoubtedly jack up the sales and general revenue of Infinite Warfare, so selling it separately would work against that plan. After all, Activision/IW don't give a shit if their fans get their hands on MWR at all, all the game is is a marketing tool to move copies of their new title through bundled nostalgia.
Also this explains why there are only 10 multiplayer maps included with MWR if it cannot be sold separately: they want to bundle it with Infinite Warfare DLC for the same reasons as above, to bundle nostalgia with Infinite Warfare to once again generate sales. Honestly, they'll probably withhold classics like Overgrowth for DLC to help add to that incentive. It's sort of the same way Treyarch DLC works with Zombies.
Now once initial hype for infinite Warfare dies down in a few months and Activision sees the sales it wants, then they'll release it digitally. This will probably occur after the DLC rotation has been completed so they can create a complete bundle and sell it for $40, possibly with physical copy hype.
Calling it now.