On the contrary, the US produces big TV viewing figures and a huge number of Americans travel to every world cup. It is true that, as a proportion of its population, the USA's participation lags behind that of the traditional football powers, but football has a much bigger hold on the USA than mainstream American culture might suggest.
This is partially because, in US culture, football has long been the sport of immigrants - and with immigration a topic of major political debate at the moment in the US, supporting the sport could be seen in certain circles as a very political choice - it goes without saying that everyone knows that Latin America is both a football-crazy region of the world AND the US's biggest source of immigration. Additionally, some Americans like to depict soccer fandom as the province of effete, snobbish, urban bourgeois hipsters. This insult largely targets white middle class soccer fans, but ignores all of the immigrants and children of immigrants who are more likely to be working class, and at any rate never saw football as a snobbish sport - it was the ONLY sport. They know nothing about that deal where you wear silly helmets, throw the egg-shaped thing around, crash into each other and dance every few minutes between commercials.