.................... So, without getting political, why is the culture of the United States of America so fixated on capitalism?
I believe the historial context of where capitalism originated, and what drove it have not been considered on the thread. Unless the origins of a philosophy (any philosophy) are taken into account, it is hard to quantify how or why that philosphy evolved into whatever it maybe in any point in time. To jump into the middle of a constantly shifting evolution of systemic thought, or change in the evolution of a philosophy lays such thinking wide open to mis charactorisation - even plain deliberate untruth to conveniently skip over historial reality in favour of popularist thinking. It all revolves around the evolution of the Rights and Freedom of Individuals
The whole principle of the "Freedom of the Individual under Law" - a pre-requisite for any "Free" system , beit legal, commercial, trade, private way of life, etc - was first encapsulated by the Magna Carta in 1215 (without getting picky over year of origin and revisions etc). It was as a result of the Barons in England rebelling against the absolute power of the monarchy, and the whole framework of feudalism, serfdom etc that had spawned for centuries. The Magna Carta laid down the defining principles of Freedom at its basic forms, so much so, that over the following 200 years European Countries based their constitutions, political and legal system on various variants of that original charter. Whilst in practice Feudalism still existed in many ways for another 300 years or so in Europe post Magna Carta, it days were numbered, the Magna Carta had "let the cat out of the bag". Absolute Monachies were increasingly citing the Magna Carta throughout Europe as their justification for appearing "Liberal" and "acting for the people". The principle of the Constitutional Monarch was born, and spreading like a wild fire.
The importance of it, and its key role in the evolution of Democracy is strongly recognised by the American Bar Association as the foundation of the current democratic and legal sysems throughout the world. The American Bar Association paid for, and maintain a memorial on the Site of the signing of the Magna Carta in England. See: [url=http://www.runnymede.gov.uk/portal/site/magnacarta/menuitem.f38b9c56ee32432edf7a8e7c9f8ca028/]American Bar Memorial[/url]. My personal interest? I live 4 miles down the road from the Memorial.
The Magna Carta has even been recently Cited by the Californian Supreme Court as a relevant factor in its judgement concerning non-attorney judges (lay magistrates in UK terms) in its 1974 Ruling:
".............. The principle we announce today is not a novel one. It dates back at least to 1215 and the Magna Carta (§ 45) where it was written, "We will not make men justices, constables, sheriffs, or bailiffs, unless they are such as know the law of the realm, and are minded to observe it rightly." We conclude that, under today's advanced standards, due process demands that henceforth a defendant charged with an offense carrying a possible jail sentence must be provided with an attorney judge to preside over the proceedings, unless he elects to waive such right"
In the Centuries that followed the Magna Carta, Freedom for individuals was fought for, argued over, evolved, even caused many major European wars. It was subjected to ever increasing philosophical thinking in terms of Free Markets (Europepean source of angst in 16th and 17th centuries, and had spawned the French Revolution with its crys of "liberty and Justice", the last vestige of Middle Ages Feudalism ended up in the guillotine basket along with the head of the French King. The rise of Liberalism as a political mainstream grew alongside Free Marketeers in Europe in the Middle Ages, and was spawned by such thinking.
In the swirl of these events, and growing clamour for Political and Personal Freedom, a group of colonists gathered in England, driven by a hatred of the last vestages of feudalism and absolute monarchists epoused by King George. The situation was a complex one, however the aspect pertinent to the OP above, is the passionate and driving desire by the colonalists to be rid of interfering external influences in their lives - they wanted freedom, genuine unadulterated Freedom, the Magna Carta was about to spawn its ultimate proponents. So they sailed from UK for the North American Continent.
Here arguably, was Utopia, or as close as you would get to it at that time. They were the founding colonists, the first, it was a clean sheet, no external influence existed to hold back their passionate desire to place the individual and their freedoms above all else. So much so, as we all know, they booted out King George and us Brits after unjustified interference from King George in the colony's affairs. The US Independence, together with the heavy influence of the prior French Revolution and attendent French Republic Constitution, let loose an unrestrained surge towards individual freedoms, and free trade principles in North America. Whilst such thinking had spawned in the Europe, it was grossly constrained in its early days by the last of the Feudal Kings. Not so in the North American Continent.
A famous quote from Churchill:
"Democracy is the worst form of government, except all those tried before it"
The philosophies of Democracy, Free Market, and Liberalism exploded outwards in the US like a Bush Fire, there was nothing to dampen it down. No external influence as in Europe, existed on the Continent to hold it back. It was driven by that burning desire of the very first founding colonists from England to create a life guided by Freedom in its ultimate forms. Its but a short hop to evolve the US version of capitalism from such an historic background.
So the short answer to the OP is "its all they have ever known". If such principles are all a population has ever known, has survived over 250 years of internal evolution of ideas of its practical incarnation, and many external threats to its philosophical existence as the very core of its daily life ........
...... You get real passionate about it