Because it's the only system shown through out history to work?

If there are better alternatives, why aren't other countries uses such a model? Why is the US _THE_ powerhouse, followed on by other highly market driven economies?

People "envision" schemes all the time. I prefer to live in a world where my drugs are real, not envisioned.

Because the only alternative economic system, Communism, was a total failure at drug development compared to the system we have. We have an example of a working system and another, much less productive defunct competitor.