There’s no technical reason why you couldn’t. It’s probably just some stupid marketing reason like:

• A. Monster doesn’t want to sell its concentrate this way.
• B. They do, but not at a price that would justify including it as an option among other common self serve beverages.
• C. Stores don’t want to offer unlimited access to dangerously high levels of caffeine after a Panera customer with a pre-existing heart condition drank about a gallon of caffeinated lemonade and then dropped dead in the dining room.