In-universe explanation - Uncle Owen instantly deduced that 3PO's memory had been wiped, which he knew to be a common practice, when 3PO didn't appear to recognize him. He also, in the short time available, couldn't come up with a plausible reason to pass on 3PO that wouldn't raise questions from Luke. Perhaps this supposed plot hole is really a brilliantly subtle piece of writing that went over the heads of many. (no, not really)
