I think the issue is actually that it is hard to have a universal device that could determine the volume, so the weight option wouldn't work. A half empty bottle should weigh less than a full one of the same size, but the machine wouldn't know that one is half full.

It makes sense that an ultrasound or some other radio-based density check would be pretty accurate. Of course, since the TSA has some special secret crazy liquid threat other than the binary bomb that they can't tell us about, they can't let on that it isn't water based or we might be able to figure out how to make one and get it on a plane

Too bad this will never make it here