Why do devices fail?
I would say for the majority of the time - physical damage or water (moisture) damage.
Any fire alarm device that is physically damaged - a pull station that got smashed with a cart, horn strobe that someone stuck a pen into to silence it (see that in hotel rooms mostly), a smoke detector that got painted, strobe missing the plastic lens - must be replaced.
Water damage - probably the #1 killer of smoke detectors! Roof leaks, plumbing leaks, overflowing toilets or bathtubs, or just moisture getting into an outside wall, will all destroy a fire alarm device. I’ve pulled down devices with rusted out screw terminals with no clear indication on where the water came from. Pull stations on exterior walls are good for this. But the best one was a daycare where the smoke detector in the basement got wet from a toilet overflowing - I simply cut the wires going into the detector, relocated the new detector a couple feet away, and left the old one in place still full of “water”. I wasn’t touching it! Told the building owner they had to take it down!
There can always be no real explanation for the device not working. I’ve have horn/strobes just not work! I would imagine there is some small component in the device (diode, transistor, capacitor) that just failed due to a voltage spike or poor manufacturing, and that is causing it to not work correctly. And with more of this stuff coming from China, I’m sure the failure rate is going to go high. Funny, because it seems like those old 7002’s just won’t quit!
Dust and dirt can be another factor. Smoke detectors that get too dirty will fail to operate correctly or give a trouble signal to the panel. And you figure most devices are not environmentally sealed, especially on the back sides - you get a horn/strobe mounted on a plenum ceiling, even with a back box, that’s still a lot of dirty air moving through that area. Look at the BG-10 recall - when they were first put in, they worked just fine. Today, the buttons are getting all gummed up and not working. Yeah, you could say it’s a simple switch, but that’s just it, it’s a simple switch! Those contacts internally get dirty, that pull station isn’t going to work.
Another thing to consider too is that fire alarm systems get inspected yearly. And when that device fails, there’s a governing body to follow up on that failed device (local fire marshal). So there’s an incentive to replace that bad horn/strobe - especially when you as a building owner are paying money for the building, and if the building gets shut down because you failed to fix the problems, you are losing that income the building generates. And things in buildings get replaced all the time - HVAC systems, electrical systems, security systems, etc. You just don’t see them as much because they are hidden above ceilings or mechanical rooms. The fire alarm system just seems to be more “visual” in the building.