Most municipal governments are funded by either sales taxes or property - based taxes. Small towns generally don’t have the tax base necessary to fund fully paid “professional” public services. As construction methods and sprinkler systems have evolved, most large municipal fire departments have become emergency medical departments that also put out fires. There are cheaper ways to provide emergency medical services by contracting with private ambulance companies. So the number of fire responses doesn’t really justify 24/7 live-in union firefighters.
There are also, incidentally, a few locales where firefighting is provided by private agencies, and residents pay a yearly membership fee if they want the big trucks to show up when needed.
Also, in the middle range, paid/volunteer departments are common, with one firefighter at the station, and volunteers providing extra manpower as needed.
A lot of small departments are special districts, funded by a fire suppression-specific tax on local property owners. Those local districts are often the main political entity for the community. I’m familiar with a small district that contracted with the neighboring municipal government department to provide all their services, yet regularly voted to maintain the district because it was the seat of representative government for their area.
West Virginia funds fire departments through an insurance surtax. All departments in the state receive the same amount which is a bit more than $40,000 per year. That’s not enough to run most depts. which means local fund raising and in some counties tax levies provide the rest.