Monopolies are prevented, or rather restricted for 2 reasons:
The larger the company, the harder it will be to control. It will fall apart eventually because of lack of teamwork, greedy people, etc.
High prices and low quality will make room for new competition. People would buy the cheaper, better products from other companies. And these companies will be forced to create innovative ideas to grow.
If monopolies are prevented and fall apart by themselves, then why did the US government step in to break up the monopolies of AT&T and Standard Oil? Should we have simply waited until it broke up by itself? Is it worth waiting years, potentially decades for big companies to abuse the market just because "it'll break up by itself"?
Have you heard of barriers to entry? It's a pretty important concept in economics. High prices and low quality will not make room for new competition, if new start-ups cannot overcome barriers to entry. If you can't afford to build up the capital, or the economy of scale, or survive predatory pricing, then you can't fight a monopoly. Good products don't spring out of the ground, they need capital and investment and the survival of the firm to form. And companies don't have to be forced to create innovative ideas when they have a monopoly. That's why monopolies are bad - because they stifle growth - and that's why governments have legitimate reasons to break them up.
A market with a monopoly is no longer a free market, because prices no longer reflect the balance between supply and demand, and therefore they don't properly signal for the allocation of resources any longer. If market forces aren't working, how do you expect the monopoly to just break up by itself?
United States isn't capitalist. Similar to how we are a Republic, not a Democracy. We are almost, but not quite. Anyways, we are heading in a more left direction. For example, Net Neutrality was recently passed less than a year ago.
You should provide definitions for all these words you're using because:
Capitalism is an economic system based on private ownership of the means of production and the creation of goods and services for profit, with a price system providing price signals for the allocation of the factors of production.
Which aptly describes the economic system of the United States, yet you say the US isn't capitalist. And:
Democracy is further defined as (a:) "government by the people; especially : rule of the majority (b:) " a government in which the supreme power is vested in the people and exercised by them directly or indirectly through a system of representation usually involving periodically held free elections."
According to political scientist Larry Diamond, it consists of four key elements:
A political system for choosing and replacing the government through free and fair elections.
The active participation of the people, as citizens, in politics and civic life.
Protection of the human rights of all citizens.
A rule of law, in which the laws and procedures apply equally to all citizens.
Which again describes very well the political system of the United States, yet you say that the US isn't a democracy.
As for social mobility (rather lack thereof), isn't that more of social expectations? And there are many other factors other than just economy such as schooling systems. And the public schools in the US aren't exactly good, and they vary on the location.
You said about socialist systems that "everyone is stuck where they are- lower class! There will be no room to even move from the lower class to a higher class." But there's more social mobility in countries with more socialist economies meaning that less people get stuck in a particular class. That's what you meant when you said everyone's stuck in the same class, right?