Toshiba -- When the sounds turn off, long pressing on 8 for 3 seconds, it sounds a long beep and the sound turn off. All the buttons has no beep when they are pressed, including the ending cooking sounds is turn off too.

That's weird! A normal oven would continue cooking the food, dishwashers need the steam to escape to start drying, clothes get creased in tumble dryers...but a microwave ? Just why?

That's every microwave that I have ever seen. I don't know the reason either.

And it's not like those machines are silent when they are running, so you would need some sound to identify that they are not...

Mine does too, so I disabled the sounds by holding the "2" button for a few seconds. Might differ by manufacturer but hopefully yours has that option.

I constantly forget my food in the microwave for hours at a time.

Let it beep 3 times maximum(!), if you keep forgetting, you probably weren't very hungry.

And yet you are still miraculously alive, despite not being alarmed of the terrifying situation

I learned recently that most of them have a way to turn off the sound. It's usually hidden and requires the manual or a web search to figure out.