He's not at the edges of the page partially due to the print process. You need a large border of disposable material that can be cut from the edges (bleed) should the alignment of the pages be slightly out when the book is bound and trimmed. It wouldn't be much finding half of Wally, so they don't put him where he might be cut in two with a guillotine.

(He's still better as Wally than Waldo.)