If every update required a manual power cycle, almost all of the internet would be off, most waiting for one of the far too few technical staff to cycle it back on manually.

Our phones wouldn't wake us up in the morning, as they too would be off.

Power-up is hard on hardware, but restarting is often needed for software reasons, so the software makes it an option.

Shutdown can be a lengthy process.

Bootup can be a lengthy process.

Automatically is the fastest way to do both.

And get back to work.

Yes, we used to do that, back in the dinosaur (286-486, DOS and Win3.1) days.

“Have you tried turning it off and then on again” became such a common response to Windows 95+ being kind of crap that they started to include the option for convenience.

On Windows restart is not the same as shutdown/power on. I had a pc thar could not restart, but could boot after shutdown.

Is that too many options for you?