It isn't. But that's not an argument for nationalizing AT&T; it's an argument for privatizing the others. In fact, two out of three of them already have private competitors in the US; the only one that doesn't is Amtrak.
[1] https://www.fool.com/investing/general/2013/03/04/how-the-po...
So what? That has nothing to do with the fact that private companies have proven, through the entire history of the United States (including before the USA was even an independent country), that they can provide postal service. So there is no need for the government to be doing it.
> as for the TSA, it used to be private, do I really have to argue why that was a bad idea?
If you have such an argument, you should make it to the airports that use private screeners today.
Sure!
Um, what? Rent seeking is one way of taking advantage of government-granted privileges. How are FedEx, UPS, DHL, etc. rent seeking?
Edit: instead of downvoting me, you may want to argue why AT&T is any different than Amtrak, The Postal Service, and the TSA.