"Good design" is in the eye of the beholder. For example, personally, I find Apple's designs to be a bit contrived, pretentious, and having the feel of trying too hard to be simple. However, a lot of people don't agree. In particular, people who: a) have a lot of disposable income or b) who want to attach themselves to a "sexy" technology brand probably either don't even think about Apple's designs, or else find them attractive or c) both of those.

In other words, they became the most valuable company in the world, even though they have a tiny share of the market in almost every segment they're in other than the mobile handset space, because enough people are willing to pay the "Apple Tax".