Boilers heat water rather than air and send it around the house to radiators, baseboards or loops in the floor. Fewer companies around here want to touch them, which is a large part of why we do. Installs, replacements, service, and the repairs other outfits would rather pass on.
A lot of companies in this trade are furnace and air conditioner companies. Boilers are a different skill set, the parts are less common and the diagnosis takes longer, so plenty of outfits quietly decline the work or quote a full replacement instead of a repair. If you have been told your boiler is beyond serviceable, it is worth a second opinion before you spend on a new one.
The other honest point: annual service on a boiler is not the upsell people assume it is on a furnace. A system full of water, under pressure, with pumps and expansion, degrades in ways a forced air furnace simply does not. The systems that last decades are the ones somebody looked at every year.