Because they're nothing great than a fast DDR2 800/1066MHz ones.
However, if you already have a DDR3 board or have finalized in getting a DDR3 board, then there's not much to say against a cheap 1333MHz DDR3 ram. If you had mentioned going for a 1600MHz DDR3 ram (CL7 or CL8), then I wouldn't have asked you the question.