Hi,
Welcome to the eForum.
You are not the first to question why an account holder should pay for a device to supply signal when they are already paying a monthly tariff for a service. Every Network has fringe coverage and Not Spots so strive to try and offer a viable alternative.
The hardware will have a manufacturing / materials cost.
Every network generates revenue to pay for Hardware and Software such as the App's you mention other Networks offer. Because these are not integrated into the phone they do have issues.
As mentioned Calling Over WiFi is on its way which I believe is the way forward.
Current Phone >
Samsung Galaxy s²⁵ Ultra 512gb.