Its probably because its widely known that chess computers are always found guilty of engine use.
Maybe there is no satisfaction for them to beat a computer, it is strange you dont hear of more challenges, I doubt whether it is because they are afraid of losing to a computer though but who knows.
Legendary Garry Kasparov used to challenge with Chess computers and it was highly publicized (Played with Deep Blue).Similarly Kramnic played with Fritz . I believe this is one of the reason chess became more interesting.But I wonder why other top players are not playing with chess computers at open event? Are they afraid of loosing?