I always figured the main reason was that the hook isn't designed to have enough saftey to make sure the climbing rope or friction saver doesn't come out of the hook.
Just like biners for climbing have to have the 3 way movement to open them, and snaps have to have two movements to open them.
I know some hooks on cranes have nice saftey latches, but some of those break over time and some cranes don't have them at all.
I really think this is why.
they wouldn't care about burs or wonder if a hook is cracked. Shoot, the cable clamps or cable or boom for that matter could be cracked or damaged.
It's gotta be because a rope could come out of the hook and kill a climber.
Much like happend a few years ago here in MD.
hispanic climber, hot dog crane operator, hoisted climber, climbers climbing line in the hook. Hot dog operator sends the poor guy down through the brush of the crown, ball inverts as climber or line is stuck (crane operator not likely paying attention), climbers line comes out, climber falls to his death.