Well,
The centerfielder does back up the throw to 2nd base.
But...he can't sprint to 2nd because it's not like the throw goes straight to center field all of the time. (or any base, really)
Often a ball could bounce off the bag, bounce off the runner, it could take a weird hop and skip left or right, the throw could have a bit of a curve to it...
Even if the catcher's throw is just a few feet off, the wrong play by the center fielder makes a stolen base a virtual automatic 2-base error and a run will score.
It's just not worth it to get that close to the bag while backing up the infielder.