They're designed for impacts in mostly one direction. Your knees can run just fine, but a kick to the side and you can easily destroy them.

Sure, but the same bicycle that lands you safely from a 5 meter drop on a mountain trail will get bent and useless if dropped two feet over a rock at the wrong angle. Bicycles are a compromise between material lightness and structural strength. Carbon fiber cracks and aluminum bends if you apply force at the precise angle, and it doesn't take too much to ruin a bicycle. Hit a curb at low speed at the wrong angle and you can ruin a frame forever. Sure, the bicycle will still ride, but the balance will be all out of whack from then on.

Also, in most of the world, guarantees don't cover shipping. So any damage is the responsibility of the courier, and good luck with that claim. The alternative is costly shipping insurance for high value items. It is not like couriers are gentle with TVs, it's that they have the same misinformed beliefs as this comment and are rougher with other items they incorrectly perceive as tougher.

If just changing the packaging significantly reduces property destruction, then the problem wasn't the bicycle's durability.

That's why I don't trip if my electronics are handled a lil bit rough in transit. I rather they break in shipping so I can return them and steer clear of that company than later because they're flimsy shitty products. That said, I understand some electronics need to be handled with care.