It's excusable because it's just tasty food, which theoretically we are supposed not to eat too much. The problem is that this food became more tasty, convenient and sometimes cheaper (at least in time-value) than normally cooked food and we as a society don't even have enough time and knowledge to cook proper healthy food.
> (and nothing else)
Because it's cheap. There are many places where you can buy healthy food but it's more expensive.
(Yes, subsidies are necessary, national food security etc.; but a LOT is just from lobbying and corruption. Never gonna reduce them because of that, but we can dream.)
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.
I also think it's a scale thing. Selling grandma's cookies at the grocer? Fine. Megacorp optimized junk (and nothing else) sold at every 7/11 on the continent? Perhaps not so excusable.