Because if there was a friend in that house before the murders, LE would know that. There would be evidence of a 4th person in the house, in the ways I described earlier. Beer cans, drinking glasses, snacks, blankets, footprints, fingerprints, etc. The scope of their investigation does not at all point to that being the case. They did two searches, one with 100 people and one with 2,000. They released photos of cars of interest in the area, two possible cars, not CERTAIN cars, but possible cars. This is evidence of them finding leads, finding more things to go on to narrow it down. If they knew it was a friend or family member, someone who knew the family well enough to spend time there hanging out late on a Sunday evening, the investigation would be much more focused.
It's not that. I virtually guarantee that is not what happened here.