'Cos it's shit and weak?.
And while we're on the subject of beverages - both coffee and wine are the Emperor's new clothes, you pay some con-artists an arm and leg for their products so you have to go with the "oooh isn't this so good much better than the shite them oiks drink". Fuckin' nazis.
You want Tea and Beer not dried poo nuggets and stomped grapes.
Wine has a collectors market associated with it, where there is little correlation to taste vs price, to a point.
There's some of that in the single estate coffee commodities market.
But, like anything, they are a craft. There's an understood way to prepare them correctly. It's not particularly complicated. You pay the same amount to make it shitty, or buy a shitty coffee, that you do to get a great coffee. Sometimes less. You pay a little more to get better beans, but not necessarily, and mostly its just knowing where to get beans that are freshly roasted.
There's definitely a price difference between Dunkin Donuts, or other mass produced coffee, and small batch brewed coffee made preperly, just like there's a price difference between Magners and a decent scrumpy, or budweiser vs a cask ale, or flowery orange pekoe assam and some 2 year old dust in a bag. It's not an invented difference. There are errors being made for the sake of production. Just like I can make a sausage egg and cheese sandwich that will shit on something you can buy at mcdonalds, and it's not because I'm a brilliant cook.
If you think those price differences aren't worth the additional taste, that's fine, but if you think you can't discern the difference between quality on that broad a scale, then you have fallen sway to some food related moral relativism, where everything tastes the same and nothing is better. And that's just wrong. And if you think that, then you're wrong. Yea, people go overboard, yes there's social status and identity bound up in this shit. But there's also some very basic chemistry.