Let's compare it to something more relevant... for my in home internet, I pay a fixed monthly fee for unlimited data.
But it is different. For example, with home Internet you also buy data at a fixed speed rather than fixed amount. Slower speed packages cost less money and slower data means you use less of it. Infrastructure costs are also lower for home Internet because the cable and fiber lines are largely one time costs for the provider. They don't have to constantly upgrade towers with each succeeding generation. Also most home Internet providers bundled in several products with Internet so they recoup costs that way as well. If you look at the cost of an Internet-only home plan either via cable or the telephone company it's usually a lesser value than if you also bought TV and/or home phone service with it.