They have to pay their employees and make a profit too. A mocha is more complex a drink compared to a cappuccino and thus they can charge increase their profit. They are in business to profit after all. While I have no idea about the prices at Starbucks, I can tell you that espresso is quite labor intensive to do correctly and the equipment is expensive too. A commercial espresso machine can easily cost over $10 thousand and the grinder can be a thousand or more. Even then, they need to calibrate everything like dialing in the grind size, tamp pressure, and timing so that the product comes out good. Too slow or fast or too coarse or fine and the shot is not good.

I think I read once that brewed coffee sometimes does not actually make money and sometimes is sold at small loss and that more complex drinks are where the money is at.

I have read that Ritual sells a $6-7 cup of coffee which they brew to order. I was speaking to a barista at a local coffee shop that's been around for maybe 2 or 3 years that only recently started to sell coffee and he said they settled on French press because it is far less labor intensive than a Hario V60 which they considered initially. They only brew to order so nothing is pre-made and if they went with a V60, it would not have been sustainable, in a business sense, until they got to about $6 and that would not sell well.