Except they, perhaps, don't wish to strain themselves while also contributing damage. They may also not want to expend cruor. Clearly there's a niche for it otherwise parties wouldn't opt for one.
I don't even see why it's a problem. Let people play the way they want. You want more classic parties? Make them, nothing is stopping you. You're delerious if you think the state of partying will suddenly become exponentially better if we remove leeching. People will go back to Astral flows (also much contested, but never stopped by SE) and very few people will level a new job.