Windows 11 can run many more different UI toolkits, all jumbled together. It has more graphical effects in there. It has so much telemetry and Microsoft Defender will never-ever give up and will inspect everything, all the time.
I honestly think this is a difficult and fascinating question. This is like the dark energy of software cosmology. Why is the natural state to get larger and more complex for un-proportionate pleasure of use?
Largely, I think, because devs are given too powerful computers. It's easier for companies to "fix" or preemptively had off performance bugs by giving developers high-end computers than to spend extra development time truly fixing them.