But you can't use that "wasted" energy to propell a non-collapsing saboted round. This is because such a "much simpler" round will fragment inside a non-tapering barrel when subjected to the same level of exploding force that a collapsing round in a tapered bore experiences. Taking the path of least resistance, the expanding gases in a STRAIGHT bore will blow the lighter sabot AROUND the round's heavier core. And if you make the sabot and it's attachment method strong enough to resist that result, then it isn't a sabot round any longer. It's a normal, much larger regular round. 

The difficulty in machining tapered bores (prior to development of 3D manufacturing) plus the ability of a NON-tapered bore to fire other types of rounds is why tapered bores fell out of use.