You could use a differencing tool to confirm and update only changes but that might be just as time consuming as running the update. Also from time to time there are DB structure changes that just replacing new files would not make that change and the average user wouldn't know how to make the change or might accidentally do it wrong and break their site.
Even doing the file change difference can be done wrong if you're not careful. So even though there may be some users that can handle it there are probably many more that can not and it might open a can of worms for support trying to maintain different packages.
