The boss usually doesn't have any actual physical strength (unless maybe you're Genghis Khan or something)
Imagine 2000 years ago: A merchant prepares a caravan of goods to trek across the wilderness to a far-off settlement. He hires some guards. He sells the goods and pays off the guards a portion of the profits.
What keeps the guards from taking all the profit?
Or a landlord hires some goons to go beat up a tenant for not paying rent on time. Why don't the goons beat up the landlord and take the income from ALL his properties?
Or, after a miserly old oligarch pays off his bodyguards a few times, they'll eventually learn where he keeps all his money. What keeps them from torturing him to give it all to them?
Why didn't Bronn just stomp Tyrion Lannister? He could literally SEE the chest where Tyrion kept all the gold! Why walk around with him and risk getting injured or slain?