Examples two ways:
Locally, Bristol Virginia Utilities offers television, telephone and Internet services. I have no problem with this. First, it's not a matter of Constitutional authority, because it's not done by the Federal government - local government can do what it wants. Second, it's not a monopoly - alternative television and phone services, including traditional cable and wired telephone, are also available in the area. I use BVU's OptiNet service for Internet, because it's faster and more reliable, as well as lower cost. But I use ooma for my phone service, because they have one up-front cost for lifetime unlimited service (international calling and premier features are extra), and I use Dish Network for television because they had lower pricing for what you get (for the first couple of years, anyway; that has since changed).
In contrast, some want a "single-payer" healthcare system, effectively giving the federal government a monopoly on health insurance. This is not a short-term thing; people have been pushing for similar ideas for decades. Proponents want a government monopoly on healthcare where others are not allowed to compete - Hillary Clinton's 1993 healthcare plan would have criminalized any care by any provider that operated outside the federal system.
Similarly, the Postal Service has a law-enforced monopoly on mail service. Other services are not allowed to deliver non-priority mail, and other services are not allowed to use the mailbox - that is your own personal property - for any non-USPS use.
I received a mailing in an advertising packet that said that, if it were not for advertising, a stamp would cost $4. No it wouldn't. If a stamp did cost $4 but UPS and FedEx (and others) were permitted to compete, they would offer faster (and tracked) service for less. Right now, I can send a package by FedEx's standard ground service into other areas of my zone for $6.40; in addition, I will have an estimated delivery date before I send the package, step-by-step detail along the route, date and time of delivery, and name of the person accepting delivery. And the package is insured for up to $100 at no additional cost.
But others are not allowed to compete. A business tried once (they offered faster delivery than USPS at lower letter rates), and they were forced by the government to quit. This is not right. If others can do better, they should be allowed to do better. If they can't do better, then they should be allowed to try and fail.