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.