Because of inflation. With all that money circulating around, it becomes impossible to make that money worth as much as it was when most people weren't rich. When the value of money goes down far enough, one million dollars will likely not even be considered as "rich" by global standards.
Of course you can, everyone has the potential to be rich and successful under capitalist systems, it's about decisions and planning as much as it is about hard-work and effort. If you can succeed in the ocean of business and free markets, then you will but if not, you won't.
Think about it, the US doesn't have a 'King' and yet we have rulers. We attempt to rule by law but everyone should recognize that those with power (wealth) can (and do) shape those laws to create an outcome most beneficial to them, thus increasing their power (wealth). I guess the difference is that capitalism creates elites (rich or upper class) based on a man's ability to gain wealth (which is power), whereas a monarchy creates elites by blood and favors, communism creates elites by rank in party, and it goes on and on. All current forms of government create a hierarchy where a low and middle class fulfill the will of an upper class. Which is why the upper will always be prone to wealth (power) and those lower will hope for that wealth to 'trickle down'. Capitalism provides the most opportunity to people prone to human nature, but as such there will always be rich and poor. Power and wealth are incentives in capitalism and if intelligently applied a man can create a formidable amount of wealth, but your birth unfortunately has much to do with the opportunities presented to you.
If three people are rich, forty people have to be poor, and the other fifty-seven have to be in between. Otherwise the rich have no support. This is because there has to be a powerful enough economic engine that the rich can use it to run industries and stay rich.
If you mean the free trade that Adam Smith layed out in the wealth of nations, then yes, if you mean the commie-feudal Hybrid not in any way free trade system we have now then no, we are in no way a capitalist country, we are what you call communist, ugh its disc ousting.
Is everyone rich now? No. Do you think a mixed economy is any better than a capitalist economy. With capitalism at least the people feel empowered and can make a difference without having to rely on a government system to fix everything for them. So is everyone going to be wealthy no but every can be able to provide for themselves and there families if they work for it. And that's rich enough for me.
What is rick or not is all relative. Sure some people will always have more money than others, however I don't think that the current amount of wealth inequality is inescapable nor healthy for our economy. In fact capitalism seems to work best when every one has relatively similar amounts of wealth, with just enough inequality for people to compete and innovate.