Having come within a whisker of making a gear up landing in a Cheyenne 20+ years ago, I can tell you that it's not always black and white.
I will start by saying that we accepted full responsibility for what occurred. I was training with a very experienced captain on a new aircraft type (for him) and we were doing circuits at a controlled airport on a busy Sunday afternoon. We were using the longer runway while the tower had light aircraft doing circuits on the shorter one - even though the amount of crosswind was the same as the wind was splitting the two of them down the middle. The tower guy was spending a lot of time telling us about traffic in the other pattern - almost obsessively so and definitely to the point of distraction. I gave the trainee a simulated engine failure on the downwind and should have noticed that when the power lever on that engine was brought to idle the gear warning horn didn't sound. Due to the constant distractions from the tower guy I missed it. Because we were flying a single engine approach, neither of us noticed that he was carrying less thrust than we typically needed on an approach. As he was just starting the flare, he yelled, "!!!!! No gear!" He immediately started a go-around. Unfortunately in the gusty conditions a wing dropped during the go-around and one of the propellers contacted the runway. We were quite fortunate that we didn't throw a blade. We brought it back with the damaged engine at idle and landed a few minutes later. After landing we discovered that the blades on the "good" propeller had also contacted the ground but only just barely.
Later on we learned that the micro switches for the landing gear warning system in the power quadrant had not been set properly during a recent maintenance inspection. They were not tightened down and had backed off to below the idle position so there was no way we would have received a gear warning.
This incident occurred within a few weeks of the decision which led to tower controllers in Canada no longer saying "check gear down" when issuing a landing clearance. Like I said, we were responsible and I accept that but there were definitely contributing factors and changing any one of them would probably have led to a better result.