Because they dont want to?
So they can show paid programs all afternoon?
The last few weeks I've seen my local CBS, which is owned BY CBS, have paid programs Saturday afternoon for 2 hours until network programming comes on at say 2PM CDT.
WREG in Memphis has been terrible in recent years about filling non-sports times on Saturday and Sunday afternoons with infomercials. They already had infomercials scheduled for the 2 or 3 hours before the golf coverage was supposed to start this weekend. CBS definitely doesn't need to be turning even more time over to the local stations to do nothing but fill it with infomercials.
If it's like it usually is, there will be some weekends between now and when Football starts in the Fall that CBS will have no sports of any kind and guess what a lot of stations like WREG will fill that time with: Infomercials.
CBS, NBC, and ABC all need to come up with programing for weekend afternoons when there aren't any major sports events going on. I know it probably happens on all of them at one time or another, but bring back shows like Wide World of Sports, put on movies, repeats of prime time shows, ANYTHING to keep local stations from filling the time with infomercials!!!