That would of made a lot more SKU's for retailers to handle. It would get too complicated selling a "2gb" model or an "64GB" model. At the same time the manufacturing of seperate models would drive costs through the roof. Apple can do it because they give no option to expand the memory. If sony did that too, then the "64GB" model would be useless.
Its all about the customization on mobile gaming systems (note:NOT phones).