We could do something like that; but then you'd have a longer gap between us completing the final build (and confirming it works through beta and stress testing) and players getting their hands on it.
Way it works currently is we finish the final build; test it; then distribute the client files to mirrors and issue the release date once we know it works. Release date is set as close to the final build being completed as is realistically feasible whilst allowing us to beta test and confirm with file hosts that we can use their servers and upload the files to them.
Now, we can't make the gap between completing the build and distributing it to mirrors any quicker - that process occurs as soon as we know the build is release-stable and playable with no game-breaking bugs.
So if you want to allow a period for the community to download the update
before the actual release date, that means release would have to be pushed back by about a week, say, to allow everyone enough time to download the new version in their own pace.
You'd have to have a relative decent pre-release download period to make it worth waiting a few days before downloading - if you make the pre-release download only a day or two in advance, you'd still get the massive wave of people all hitting the download servers all at once. So, that means having maybe a week or ten days to allow everyone to download
before official release...
...which means a longer wait for you guys between us finishing the new version and you getting in-game and playing.
That's why, to date, we've not gone down that route. We figure twelve hours of slow downloads is better than twelve days of waiting
just earlier