"natural monopoly that companies in the US have over the infrastructure"
That "natural monopolies" exist outside of the mind of legislators is doubtful. "Natural monopolies" are just code for "a monopoly granted by regulatory law", probably after some solid greasing of political wheels.
For example, from The Myth of Natural Monopol
The eventual creation of the telephone monopoly was the result of a conspiracy between AT&T and politicians who wanted to offer "universal telephone service" as a pork-barrel entitlement to their constituents. Politicians began denouncing competition as "duplicative," "destructive," and "wasteful," and various economists were paid to attend congressional hearings in which they somberly declared telephony a natural monopoly. "There is nothing to be gained by competition in the local telephone business," one congressional hearing concluded.
The crusade to create a monopolistic telephone industry by government fiat finally succeeded when the federal government used World War I as an excuse to nationalize the industry in 1918. AT&T still operated its phone system, but it was controlled by a government commission headed by the postmaster general. Like so many other instances of government regulation, AT&T quickly "captured" the regulators and used the regulatory apparatus to eliminate its competitors. "By 1925 not only had virtually every state established strict rate regulation guidelines, but local telephone competition was either discouraged or explicitly prohibited within many of those jurisdictions."