Frank, presumably because that's not what the Yak pilot did. The direct cause of the accident was neither pilot seeing the other until it was too late. The Yak pilot initially called visual on the Cessna, but lost sight of it not while performing a RIAB, but while he was doing whatever he was doing in the vicinity of his house at 100 feet. The AAIB report suggests that if the Yak pilot had done a standard RIAB in accordance with his intentions expressed on the radio, the Cessna pilot would have known where to look. The A/G operator would have also had a better idea of what was going on, which understanding may or may not have influenced subsequent matters, but the absence of which cannot have helped. And if the Yak pilot had not broken off from a standard RIAB to become involved with some very low-level flying unrelated flying in the vicinity of his house, and not explained by his RT calls, it is reasonable to assume that the Yak pilot could have maintained visual contact with the Cessna.
Quote: "The Yak pilot declared his intention to carry out a 'run and break' with a time frame of 'thirty
seconds to initial'. It was thus possible for others listening on the frequency to be aware of his
intended flight path, which was to descend and make a right turn in order to align his aircraft with
the runway before carrying out the run down the runway and then break right to join the circuit
downwind. By making the left turn and descending to such a low height, he deviated from that
flight path and placed his aircraft in a position relative to the Cessna, which was not expected by
the Cessna pilot or by the radio operator."