I never understood this kind of thing. Why have a release date x days away, if you're going to have a separate date where the game is complete and you can actually download it, but not actually play it?
For many, many reasons.
First things first. Video Games are always ready MONTHS before the release date. After the development period, developers make a series of "Release Candidates", which are essentially finished versions of the game which they submit to various console publishers for certification; the games are checked against TRCs (a list of requirements that each console publisher uses to ensure that each game meets a minimum standard before they greenlight for release on their platform officially, games that fail the TRCs and release are not allowed to be sold on their platforms officially, and hence won't have the correct logos and won't have the $10 console fee).
Eventually the game meets the critical TRC items, and the game "Goes Gold" (in the old days, the final release of a game was printed on a gold master-disc which would be used to print all of the discs for the public). Once the game is gold, it then has to be printed onto discs for retail release, and any special editions have to be packaged if they haven't been done already. Meanwhile, especially nowadays thanks to the Internet, the QA teams continue working on the current RC/Gold copy of the game so they can find any crucial bugs to be eliminated in a day-one patch.
Since the game is finally ready, especially in big AAA publishers, this is the most crucial time for the marketing department, since they have access to all of the game's content for their promotional material, and they're closing in on the date of release. They're not too early so interest will wane off, and they're not too late that sales don't matter any more. You'll notice that a lot of studios use the last few months to push their game hard.
They're not done yet. The tech support lines need to be established and tested, including creating the generic scripts and help advice for what the developers/publishers expect will be the most frequently asked questions during the first week. If the game is multiplayer, servers need to undergo more stress-testing and any final hardware installs and maintenance needs to be done pre-release. All of those discs that were printed now need to be shipped worldwide, and developers need to test that their game works through the digital storefronts (Steam, uPlay, Origin etc).
A few other reasons:
A) The game's release date is set long in advanced. As well as the retailers and support teams, journalists, marketing/advertising departments (such as the people who print ads on the sides of vehicles or at bus stations) and many other people spent LONG periods of time preparing for that specific date. Moving the date forward or backward is an extremely costly ordeal and can screw over many people in the process.
B) Games that can be pre-loaded generally are 15+GBs in size, and if you allow mass-amounts of people to download at once, it's going to stress the download servers for everybody and cause network blackouts. It's best to stagger downloads and allow a large percentage of people to download early, so that when the bulk of people come online on release day, you have a large portion of people who won't be downloading as much.
C) Those final days can be valuable towards putting together a final Day 1 patch for any last remaining bugs.
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