Man, you have no clue what you are talking about. Steam doesnt need to make exclusivity arrangements because they have an unprecedented stranglehold on this particular market. There is no other storefront/launcher option that is like steam and comes even close to the massive size of its userbase. For many devs, if they for some reason cannot get on steam, its a death sentence and easily makes the difference between a potentially very popular game and game that is completely unsuccessful.
Also, Valve has proven time and time again that their own managment of Steam is often times very lackluster. The Epic Store is for the first time ever something that resembles an actual competitor to steam. And even then, its got a long way to go.