Ultimately, there's no strategic advantage to be gained by entering an industry where the barrier to entry is so low that everyone can do it. When you're competitor N+1 in a market like that, your only hope is to compete on efficiency (i.e. the Dell model), or novelty (i.e. the Apple model). Otherwise, you're just a guy in a strip mall, selling beige boxes; you might make a living from it, but you're not going to get rich.
In short, don't think of the PC industry -- think of the teriyaki restaurant industry. It's cheap to open a teriyaki joint, and anyone can do it, but unless you're going to serve gourmet teriyaki, or find a way to produce it for less (/shudder/), it's not the place that you want to be as an aspiring entrepreneur.
Obviously, one of the ways to get rich is to create a new market and exploit it. But that's probably not as easy as building the better Teriyaki restaurant, judging by how rarely lucrative new markets emerge.
They are not the same thing.
(Seriously: I am not a pessimist, but compared to some of the wild-eyed optimism that gets thrown around here, I may sometimes seem like one. My world view is that, given a choice, you should live life in a way that maximizes health, happiness and satisfaction, not money. Start a company because you want to work for yourself, not because you want to get rich -- because you probably won't.
If your whole goal when starting a project is to achieve fame and fortune, you will go through life in an almost perpetual state of disappointment. If, instead, you make choices that allow you to live a good life now, you'll be happier by definition.)
The fact that some markets have matured and been commoditized hardly implies that there is no new value to be created.