The only reason Coke and Pepsi can't do this is because they are beholden to their bottlers and distribution channel. The actual cost to make the soda is in the pennies. If it were to cost $.25 for a 2L people would be REAL fat....
You can buy Coke & Pepsi syrup for home soda making systems. A friend of mine has had one for years. It delivers carbonated soda to a spout on his sink just like it does in a bar.
Hey I call that a thing of beauty . No more having to open the fridge and take time going "ummm pepsi or coke" instead just go "I'm having this one now"