Why does apt-get not require restarts, where the update manager does?

Because apt-get (or plain simple apt) is for the pros and when the pros do a:

sudo apt-get dist-upgrade

and see a new kernel being installed, the pros know they have to reboot to activate it.

The automatic update is for everyone else out there!

Alternatively, you can now install Ubuntu Live kernel Patching from version 16.04 onwards which allows you to update your kernel while your server is running.

:-)

Update manager suggests to restart when kernels are upgraded or some other system packages with services that are marked for reboot when upgraded.

In this case /var/run/reboot-required is set by a package installation script.

Command line apt-get upgrade does not suggest anything, because it is targeted to more advanced users that can decide themselves when to restart the system ;-)

But after upgrade using apt-get Update Manager in a while will suggest to restart too.

When you install a new kernel you have to restart the machine in order for the changes to take effect, apt-get just doesn't nag about it.