The heads were very lightweight, and defragmentation algorithms were supposed to make sure that the heads seldom had to move far. Far more energy was wasted spinning the platters. And the benefit of the old system was that you only needed one system for ultra-precise positioning of the heads. This was probably not just simpler and more compact, but also more reliable. In a multi-actuator design, the failure of any of the independent positioning mechanisms kills the drive.

Not saying that this isn't progress, but 3.5" and 2.5" HDDs were a true marvel of engineering when they came out, and maybe we needed a couple of breakthroughs in electromechanics to get to MACH.2.