For no reason at all. The main requisite of the GPL license is that if you do some modifications to a GPL licensed software you must give back that changes to the community, i.e. publish the modified source code (only of the GPL licensed parts, not the whole source code of the project). To me it seems reasonable enough.

Apple not wanting to include GPL software doesn't make a lot of sense to me, since they still release the source code of the low level components as the Darwin project. And still they include some GPL software.