To put it simply.....

Most tech's have a drawer full of "known good" sensors. It's much easier to switch out a sensor to see if that's the problem than to dig into a very complicated fuel or ignition system. Sensors are typically very easy to get to and switch out for a good one. Most mechanical parts will make some sort of noise, smell, or otherwise give some indication when they are failing. Sensors are just another electrical item...... when they go they just go.