You must live in a filthy, run-down part of town.
All the Wal-Marts I've seen are very clean, though they are crowded.
Come to Houston. I'll show you a Wal-Mart just on the edge of the ritziest neighborhood in town that fits my complaint exactly.
Houston Wal-Marts are nasty across the board, which is why I can't speak for Wal-Marts elsewhere - I don't go out of my way looking for dirt.