It's allowed for two reasons.
First, if you suggest that any company's monopoly be returned to the people, who liscensed it to them in the first place, then everyone thinks that you're Stalin. Yes, being against the government granting a private party the authority to come dig up your land, run lines under them, and take profits from your land without paying you a dime of rent, is apparently the red Soviet menace incarnate. So reason number one, Americans like to lick the boots of their elite. Or ruling class, or business leaders, captains of industry (that one's a laugh), or whatever you want to call them. Wish I could figure out why.
Second, even if hypothetically the American people decided tomorrow to oust the monopolies, that still doesn't mean the net would be saved. Going from your example, if you don't want car companies owning gas stations, then a lot of people will see you as a God-fearing capitalist who opposes vertical integration.
But if you want your pipe to just be a dumb pipe, then you're one of the horsemen of the infocalypse. You must be guilty of something. Citizen, this tranmission is using non-standard port numbers for all civilian purposes, and the Ministry of Internal Security has been unable to determine the purpose of these transmissions. Also, they appear to be encrypted. Do you have an encryption permit and a valid business use for encrypting these transmissions?
Yeah anyway, there's of course the obvious solution to this crap. New amendment:
All monopolies, including, but not limited to, utilities and intellectual monopolies, granted to private parties by any government within United States territory are hereby rescinded. In the case of utilities, all authority vested in any utility established by a grant of monopoly shall hereby be vested in the government which granted it. Henceforth, no government within United States territory shall have any authority to grant any monopoly to a private party.
Anyway, we as a nation are going to have to go a lot further into some kind of Stalinist/Maoist hybrid before we realize (and have the balls to admit) that we've made some big mistakes. We can't have any progress until then.