Because they know that they'll be praised for it and probably manage to sell a couple of copies. It also makes sense as something to put on a higher-ed application or CV ("I can already write commercially successful pieces of software, I got X comment from Y developer and sold N units").
That said, you're probably right in suspecting that this is partially driven by someone other than OP. The landing page of MakeGamesWith.Us has Cheese Miners on the front page along with the text: "It's so easy we got high school students to do it".
However, regardless of OP's motivation, it is well worthwhile applauding them on a job well done at such a young age. There are nowhere near enough teenagers taking up programming, and I hope that success stories like this will encourage more young people to at least get their feet wet with this financially and mentally rewarding craft.
: This is especially troubling given the ubiquity of free development environments and online instructional material.