I assume you're talking about the safety beams that prevent the door from closing on obstacles. Have you ever had the door go down on command and then halt, without any obstacle in the way? They're very noisy sensors - lots of false positives, not the sort of thing you want to wire up a massive bomb. They also actively emit light, so not very stealthy.

The first time I saw the results of an IED blast was a day after I arrived in Iraq. I had to shake a tree in order to retrieve the trousers of the guy who blew himself up planting the thing. That happened a lot. Making IEDs isn't very hard, but making a lot of them without eventually blowing yourself up is.