Why isnt cal mag needed when growing in soil?
The layman's answer is: Soil contains and holds nutrients naturally. In fact, there's calcium already in it. When you add the lime, it contains BOATLOADS of calcium and some magnesium that will break down in the soil as you water as time goes by, so the soil will never be short of calcium, so you don't need cal mag. In a worst case scenario, you might have to water once with a half teaspoon of Epson salts to add some magnesium, but that's about it and it rarely happens.
Hydropnic grows have no medium that holds on to any nutrients at all. The roots given only those nutrients that are in the solution. The typical solutions are never going to have enough calcium in them, and magnesium is also iffy. So a cal mag regimine becomes necessary due to the fact that there is always a need for calcium and magnesium over the plants life and the standard nutrients do not contain nearly enough of either.
That's why cal mag was made to begin with. Yes, it's expensive. Hydroponics are very expensive...about 25 times more expensive than soil grows. If you can't afford it, don't do it. It's that simple.
I grow for only my personal use and found that hydroponics simply aren't worth it at all. On a larger scale, yes. It makes a hell of a difference. But if you're only doing two plants for your own use and maybe a friend or two, you're wasting your money even bothering with hydroponics.