Because Wal-Mart can't figure out inventory.
If Wal-Marts web site says a product is available for local pickup, the odds are pretty good that it's not.
I know this from experience --- and from Wal-Mart employees who tell me, "Don't believe the web site".
The useless trip to Wal-Mart costs me more than Amazon. So I just buy it on Amazon and avoid the exercise in futility.
Someone wake me up when this changes.