Those aren't called milk because they're a substitute for anything, they're called milk because that's the English word for a cloudy translucent liquid. See also milk glass, milk thistle, and milk of magnesia.

Speaking for myself, I drink almond milk because it tastes better than cow milk.

I don't know anyone who drinks non-dairy milks for concerns over carbon emissions. It's always been one of or a mix of dietary concerns, taste preference, or avoidance of the awful dairy industry.

Also, where I live, dairy comes in those tetrapaks, too.

Usually, they want a substitute because they like foods that taste roughly like milk but they can’t drink milk. Is that too much to ask?

but none of the vegetable "milks" taste anywhere close to mammalian milk.

So? Skim milk tastes nothing like full fat milk, but still has a right to exist.

It's something sorta similar to milk that can be used in roughly the same applications - lighten coffee, moisten cereal, baking, sauces, fold into mashed potatoes, etc.

I prefer the taste of pretty much all nut milks to cows milk, personally.

But what else would you put on your corn flakes?

No? I haven’t tried almond milk, but rice milk tastes similar.

Most people drink soy/almond milk because they like the taste, or because their bodies aren't very good at digesting lactose.

Allergies to casein are also becoming much more common.