> Socialism is state ownership of "the means of production".
Socialism favors social ownership of the means of production; different branches of socialism prefer different mechanisms for this. These can broadly be split into top-down models (planned economy/state ownership models) and bottom-up models (council communism, etc.) It also includes models which are somewhere in between, like economic democracy (which tends to include elements of top-down ownership by the public through the state, by recognizing broader class of public stakeholders in firms whose interests must be served in addition to private shareholders, as well as elements of bottom-up socialism in empowering employees as represented stakeholders in firms as well.)
The latter seems to be the dominant (though not exclusive -- the group is diverse) focus of modern democratic socialists; Sanders seems to fit in this group as a reformist rather than revolutionary [0] proponent of democratic socialism pursuing (as is common among reformist democratic socialism) social democracy as a incremental step on the road to socialism in the economic democracy form.
> Americans (and pretty much all Westerners) are generally uniformly pro-capitalist.
Americans and most Westerners are generally supporters of the modern mixed economy, which is as much a reaction against the 19th century system for which the term "capitalism" was coined as anything else. That said, its a reaction which superficially retains the large-scale structures of capitalist relations while greatly altering the scope and nature of the property rights which are the defining feature of capitalism.
There are proponents of capitalism in America and the West; they are economic reactionaries.
> Regulated capitalism is a fait accompli and no one is challenging it because it works.
Plenty of people are, in fact, challenging the modern mixed economy (what you call "regulated capitalism") -- from both the laissez-faire capitalist perspective and a variety of different socialist perspectives, among others.
[0] in economic structure terms, if not necessarily in terms of the structure of electoral politics and their relation to private business, the latter of which is the focus of the "political revolution" Sanders often refers to.