That's because optimization is done at the driver level, not the UI layer. I have a Surface Go 2, and was using it as an ebook reader last night. It gets over 20 hours on a single charge if I can keep the CPU in the lowest power state.Ugh. And the Surface line is a great example - stunning, innovative hardware with the exact same janky OS as on some cheap Atom-powered Chinese laptop. There is almost no advantage or real optimisation I can see from W10 on a Surface; the perceived advantage of both hardware and software from a single manufacturer is non-existent. Again, half-assed.
On the flip side, if I were to pick up a cheap Atom-powered Chinese laptop, it would have the same UI that I know, just with a less optimized driver model, resulting in more heat and less battery life. There's no need to have a different UI to differentiate between high and low end hardware.