That's because we can't all be capitalists.
It's easy to think so in vague imagination that with enough effort and passion, all workers are open to the same opportunity to become a successful, wealthy capitalist. (And that's what probably Adam Smith and he's colleagues EARNESTLY believed in). Well, let's just look around ourselves and see how many people we know have successfully made the transition to worker-capitalist.
The chances are, you'd hardly know anyone. (Unless in those really teensey-winsey small towns resembling almost closed economy). The chances are, I am a worker, you are a worker, my parents are workers, and theit parents were workers.
Why? If everyone is open to become a capitalist, everyone probably would choose being a capitalist. Everyone wants to become a capitalist. Because, they don't have to work.
If everyone's a capitalist, how would a society be able to produce profit? Capitalists are people who don't "work" by definition.
In reality, the capitalist system rarely makes a circulation in chain of events: worker becomes capitalist, he hires workers, those workers earn money, they become capitalists.. etc etc.. The circulation itself is possible, because we live no more in a feudal system. But the problem is, the procedure is way to strict and limited to say that it's "free-for-all". It works by some methods, rules and systematic insurances that ensures some people always remain as workers, while some always remain as capitalists.
And we call this "the CLASS STRUCTURE".