Because someone writing malware could destroy your hardware.

At least like this you have the option to go back to 'factory settings' (ie, a blank start) and you can expect to get your device back to the way it was when you bought it.

So you'd have no problem if a virus could physically incapacitate your keyboard, monitor, speakers, CPU and motherboard?...

In fact, back in the day it was possible to damage your keyboard through software. That must have been fun. Not.

How would you like it if a virus could flash your BIOS and render your motherboard unusable? It's happened before, and that's why most motherboards now either have their BIOS chips in sockets (so that the manufacturer can mail you a replacement) or have a back-up BIOS chip that you can switch to only with a jumper.