Hake, if disintegrate was easy to run, you wouldn't be using the cost reduction rune on Ray of Frost to make it cost almost half as much as Disintegrate. You can run ray of frost because it has a cost reduction rune AND because you have the legendary that make it not suck, but for the most part you can't sustain a spender spam unless using an extremely slow 2H with a lot of cost reduction stacked, high crit and some apoc(which with a 2H is a lot more limited since you don't get the OH for it). If you have the right gear and can afford to not use signature spells, it's probably going to be better(would still need to run a comparison with black ice vs the cost reduction rune at the expense of having to slot a free spell instead of arcane orb and use it every now and then), but that's quite a big if. It's just easier to gear for normal usage.
It's a bit like saying "I don't understand why people use a hatred builder on DH, with shrine gloves, cindercoat and helm of pride you can run cluster arrow without ever using a builder, why don't people do that". The answer should be fairly obvious.