I can’t speak for random libertarians off the street, but it’s certainly not the case that the leading theorists of anarchocapitalism (or market anarchism, if you prefer) are utopian thinkers.
These writers are of the moral conviction that voluntary social interactions are preferable to coerced social interactions. So they think that if there are methods of accomplishing an essential social function in a voluntary way, that’s preferable to the state’s handling of that function.
In any event, I don’t think it’s true that “the majority of modern-day libertarians” are anarchists. I’m willing to be proven wrong, though.