From SFO, its about a 10:30 flight, and about 8 hours time difference this time of year. A 7:00 am departure would land in LHR at 1:30 am. So, not only are no onward connections available for many hours, but you are also arriving at 1:00 am. By the time you're in a hotel, it would be closer to 3:00 am. My guess is the airlines have found this to be non-optimal and there probably isn't enough pure LHR-SFO traffic to warrant it.