As the Romans respected earlier mounds built by ancient britons and placed burials at a respectful distance in their vicinity so too the AEs in 2500BC saw the sphinx and whatever remains may still have been there left by their ancestors and thought it proper to build their pyramids at this place and tart up the sphinx too.
I know you don't agree with this idea, but we can at least debate it in historical terms. No aliens, supernatural entities conspiracies etc. need be summoned forth. And we can agree that the pyramids at Giza are 4th dynasty in origin.