It's important to note that often, these taxes aren't imposed on consumers directly, but on distributors. Distributors and retailers just choose to price gouge rather than make a fraction of a cent less per ounce sold.

Even with that in mind, it's easy to say "why do we let megacorps do it"? Well, we don't, not necessarily. They just have the financial and legal resources to impose their will. A pigouvian tax is probably the easiest tool most levels of the US government has to restrict successful vices. Look at that link again. See how much money Coke, Pepsi, etc. were spending in Philadelphia and San Francisco to fight taxes that have solid legal standing. Localities taking action need to make sure they're bulletproof legally, or else the action will be reversed.