Because FB loses control as soon a third party is involved.
This is also something that needs to be native in game.
It is also a complete nightmare concerning intellectual property because Valve takes ages to react if issues arise.
It is also a complete ripoff... Valve, not the devs, takes a hefty part of the transaction as fees.
Oh... and real world money trade is questionable at best...
It is a real boon for small indie developers with small projects but just not feasible for SBs scale.