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?

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.

Because comps have got to the point that they wont lose anymore. Computers are past the ratings of even the best now.

 No point in watching a comp go 5-0 or at best the human might get a draw as Micheal Adams did a few years ago.