Because they weren't (aren't) psychotic, cannibalistic nuts. With who they were (are) as people, you just can't get as far as the knife in the kitchen. To do so, you have to presuppose they have committed the murder and decided the kitchen drawer is a good place to hide the knife. To get to the point of them devising strategic plans like that to deal with the evidence, such as staging a burglary and keeping the knife, you have to turn them into something they were (are) not.
In the extremely unlikely event they somehow ended up being part of a scene that ended in Meredith's death, there is no way they, being who they are, would not fall apart immediately.
I know this isn't a very good explanation, but the thing is, it is not valid to start from Point B or C in this case, without addressing Point A.