this is control in the settings > ⋮ > configure updates…
uncheck the Use offline updates box.
the reason this is the default is because it forces a reboot when system packages are updated to ensure the system is operating will all the latest packages.
if you uncheck that box it means you could have updated packages installed but still be using the older packages in your runtime environment, which can cause stability issues.
as long as you are aware of the risks and take it upon yourself to initiate the reboot when you think you need to, then you can safely uncheck the box and you will see all the system packages listed in discover as separate, deselectable, items