Because the telecoms have gotten tons of help from the public, via the government, over the years. At the Federal level, the cable and satellite companies got the original anti-circumvention laws that led to the DMCA. They also get to be exempt from a lot of state level regulation that they might not like, in exchange for the far friendlier FCC. And at the local level, telcos get to use public rights of way and often have monopoly grants.
They're in no way comparable to a small entrepreneur. They're like a company that's been given (or bought) permission to operate a toll road. In exchange for being allowed to make money, they need to charge everyone the same tolls and the amount of the tolls that they charge is going to be subject to regulation.
As for why the government doesn't just build and operate the infrastructure itself ... if the telcos keep it up, that may just be what happens. Critical infrastructure has been a frequent target, historically, for nationalization when the owners of that infrastructure are perceived to be rent-seeking to excess. The public tolerance for toll roads evaporated in the 1880s [1] and they were almost entirely stamped out by the Progressives; the telecommunication operators of today would do well to keep that in mind.
[1] http://eh.net/encyclopedia/article/Klein.Majewski.Turnpikes