Why does Ubuntu want to reboot so often after installing updates?

Ubuntu will suggest a restart after the update if it can't restart the affected components in a meaningful way.

In a nutshell:

• Things like editors or command line tools don't need any kind of restart, they just get replaced.

• Services like a web server might need to be restarted by itself.

• Other things like the kernel or the core libc that affect everything will require a reboot, as they can't be easily replaced in a running system (NB: Things like ksplice exist that allows to patch some things in a running kernel).

But even with kernel updates without ksplice you don't strictly need to restart the system in every case - you have to carefully read the update notes to check if you are affected by the fixed bug(s) - if not, it's often safe to ignore this until the next update or regular maintenance cycle.

You may check var/run/reboot-required.pkgs to check which packages got recently installed and require a reboot to get fully applied - usually kernel packages.

when you have upgraded a package, the older version of that package may still be loaded into memory.

In some cases, you can restart a service if there's been an upgrade to the package containing it or to a library used by that service, but for packages like for example libc or the kernel, you need to restart. Kernel is loaded during boot and the libc is used by most programs in a typical ubuntu system.

Wether you need to restart or not depends on what you have upgraded and why.