We already know that Gantz's techniques are imperfect (see living originals being left behind, Izumi's memories surfacing, the creator of the Gantzblog's head not popping). I don't think we have any reason to believe that Gantz doesn't realize this, so offering the minimum in terms of ways to dig up pieces of the past is desirable.
Originally Posted by Youkai Katana
Kurono's decision would have made a great deal more sense if Tae's memory of him had indeed been wiped, but of course he'd have had no way of knowing at the time—I suspect the writer is going for something else, but it isn't impossible that there was originally supposed to be a gap there that was edited out for some reason.
Failure: when your best just isn't good enough.