Because presumably the GPL would force them to release their modifications. Apple gets/got away with leeching off the BSDs because of the permissive license.
A bit off topic, but is there any data or estimates of how often big companies use modified versions GPL software/libraries for their web services without releasing their modifications?
I don't know what the loss that open source suffers is in this context?
I don't think Apple would need to spend less time or money on their kernel grafted ontop of Linux 2.4 vs their kernel grafted on top of FreeBSD 4.4