Windows has had always a problem: unlike Unix, where files are referenced by an inode where the name points to, Windows has the names directly linked to the files. When Windows needs to update a system file that is in use it has to stop being used. And for some system files there only one way to do it: reboot. On Unix, the name of the file can change the inode where it points to and the system goes on. The old file exists until every process stops using it and then is erased while new processes access the new file. So no reboot is needed.

That leads to not having to update everything on reboot. Only the kernel and little more.