"Flame away!"

It's not trivial (or even possible) to do a lot of things on a package update on a running system. It's utterly impossible to safely remove or replace an ABI-unstable shared library that is dynamically loaded by running applications, for instance. The only way to do that safely is to queue up the change until the next reboot, and then shuffle the files around then.

There's a _reason_ Windows requires a reboot for many updates.

(Windows fails in that it used to just require a reboot for everything out of laziness, which made it incredibly annoying to update all those things for which a reboot was totally unnecessary. It's a lot better about that these days. Many application vendors still demand reboots after their app installs, but those are generally ignorable and are not Windows' fault.)