I can't speak confidently as to the others, which is going to make me sound like an apologist, but I know that macOS née OS X spends modern-scale processor cycles on:
(i) using the full set PDF primitives for all desktop drawing;
(ii) the dynamic dispatch that underlies its approach to UI building;
(iii) contrasted with Risc OS, the various context switch costs associated with preemptive multitasking and a full implementation of protected memory*; and
(iv) various things that were historically optional: the file indexing that goes into Spotlight, the background backup of Time Machine, the remote syncing of iCloud file storage, etc.
* if memory serves, Risc OS protects applications from each other, but doesn't protect the OS from applications.
Do you necessarily want these things? Tough, you're getting them.