Lots of reasons why installing an update is not a simple matter of "extracting some files and put them in some folder". The main reason is pretty simple: We have no idea of what an update actually does. Remember that Microsoft is generally secretive about the exact contents of each update, and most important Windows is not open source, so you can never know for sure the actual contents of an update and the correct way of installing it, unless Microsoft decided to tell.
More specifically, it's not that simple as copying a few files, even if you knew what and where. For one, most updates deal with system files, probably most constantly in use or hard to replace, or even belonging to the kernel itself. Simple copy-paste is not an easy thing for those "special" files. Keeping the system stable during the operation is difficult. The update installer uses the system account for its special permissions, and maybe (just a guess) some kernel-mode code to perform some installs.
But there is more. Updates aren't just placing files, other things are known to be affected. Updates may create/change registry entries with new configuration, can replace some configurations, create entirely new files or even create scheduled tasks (the "update" that installed the GWX malware was way more complex than just copy files, to give a concrete example).
Moreover, if you learn the API of the msi/msu installers, there is plenty of possible extension points. Those installers aren't actually "copy a bunch of files" thing, but complete programs where pretty much arbitrary code can be run. And since we don't know what actions that does exactly, we can't somply reproduce them independently.
