The core mobile business requires buying spectrum and installing cell phone towers and base stations. These are not businesses for the faint of heart, given the massive capital costs.
The payoff for this investment, though, is a competitive moat. The actual utilization of its infrastructure is effectively free on a marginal basis. Laying infrastructure is expensive, but using it is cheap.
That said, to the extent competition exists, it is quite brutal because it is zero sum; an AT&T customer is not Verizon customer is not a T-Mobile customer. The combination of scarcity in customers and zero marginal cost for incremental usage can lead to price wars.
-Stratechery