To get decent coverage you need to negotiate and pay for space, power, and internet connectivity on thousands of towers/tall buildings all owned by different people and municipalities who see you as somewhere between an annoyance and a revenue stream to exploit. Then you need a field workforce capable of performing maintenance at all of those sites (which requires making appointments and sometimes paying fees to the property owners), a supply chain of spare parts, and the RF expertise and test equipment to measure and do quality control on their coverage all around the cities you're operating in.

Then you need a way to recoup all this cost.

Being a cell carrier takes staggering amounts of money and staggering amounts of schlep. It's not for hobbyists, hackers, or small companies (who are not in fact carriers but just resell and rebrand real carriers' services). There is a reason it's the domain of giant corporations run by the kind of people who make deals (and not write code) for a living.