Picking up a guitar might be easy; playing an instrument well is hard. They say it takes ten years; I'd say it takes about that long to be a good software developer. And like being a musician, most people bail out before they've learned. A few people stick with it and still don't get good (the management promotion route is this way!).

Let's pass over the cashier thing; I was a cashier exactly once, on an exchange trip, so in a foreign language. I got the hang of it in about 20 minutes.