Why should we replace smoke alarms every 10 years?
It likely has to do with the lifetime of the hardware itself. Remember, there's a circuit board and a light source, as well as a detector. Those don't last forever. So the manufacturer certifies the device will work for only 10 years, and then (in some modern units) sets a hard sunset by using an unreplacable battery. 

In some regards, this solves the problem of people just putting new batteries into older detectors forever, not realizing the devices have stopped working (when was the last time you tested one?)