"It is not socially acceptable to ask for the person`s soul as part of a bargain, so we try to get as close as possible"
Serious answer: When working on a collab, we are spending one of the most valuable resources: time.
So one way or another, we need a reward for spending an unrecoverable resource. Money give us more time on this world because it give us the access to the resources neccesary for our survival, so is not abnormal to ask for things that trascend and/or are meaningful if money is not an option.