It's true that, even if you can brew cheaply, you could probably get beer even more cheaply by buying American macrobrew.
But it's also true that, even if you can brew beer well, you can probably buy better beer than you can make. So on neither front are you at any absolute advantage homebrewing.
Here's why I'm cheap: I like the idea that I can produce any sort of beer I want, whenever I want, at a low marginal cost. It's the same reason people buy swimming pools for their backyard. Given how much most people actually swim, it's probably cheaper in absolute terms to get a membership at the local Y or use the municipal pool. Their pools are also probably better than whatever hole you're going to dig in your yard. But some people really like the idea of being able to go swimming whenever they feel like it for zero cost. Personally, I don't, but I get the idea.
Not everyone's cheap because they want to maximize their absolute consumption potential. Some people are cheap to maximize their freedom, i.e., minimize the cost for them to do what they feel like doing. If you think about the costs of owning tools, and the value of your time, learning how to fix things around your house probably also doesn't pay off. You wouldn't expect it to, because of the division of labor: a trained plumber is so much better than you are at fixing a leaky faucet that your time is probably better spent either enjoying yourself or working longer hours. But people do DIY fix-it stuff because they want to be self-sufficient. They don't want to wait around for someone else to fix it. They don't want to think of their relationship to their home as one of managing a bunch of contract repair persons. Not for everyone, obviously, but the ethos seems perfectly intelligible to me. Homo faber ain't homo economicus--they're different outlooks.