It doesn't, I have installed many Windows updates that didn't require a reboot. Even ones I expected to need an update, like an update to a graphics driver. Screen just went blank, then came back a second later.

AFAICT it's only updates to things that run at startup time that require a reboot, probably because NTFS doesn't allow you to write to a file that's currently opened (as opposed to nearly every Linux filesystem, which handles that just fine: the process that has the file opened continues to see the "old" file, while any that open it after the write will see the "new" file — but NTFS, probably due to internal architecture, can't handle that and so you have to reboot to change files that background services are using).

Can‘t replace files that are in-use and that includes running programs or loaded DLLs. Linux can, it keeps the inode and only actually deletes upon termination of last access.