Because you run into the issue of "unscrambling the eggs". If Amazon is not organized by retail product category (and I would assume it is largely not), then it will be very difficult to split the company in that way. Courts really aren't in the business of restructuring companies and the last thing you want is for the new baby companies to fail because the restructuring went poorly.

Which employees go where, who gets what IP, what happens to all the underlying website/physical infrastructure? A "3rd party marketplace" unit is already likely smaller than is feasible for a court to oversee. By comparison, it would be a lot easier to split Amazon into "AWS" and "not-AWS" because the company is already organized that way.