You are not enough tired.

For the same reason you have to charge your phone battery every day

It can stay up as long as 3 days.

Because your Body is Tired and it tells you after 16 hours that you have to Sleep

IT can stay up for much longer than that. Soldiers regularly pull 48 and 72 hour ops.

Idk

If YOURS can only do that, might be due to your OWN sleep pattern. Your body gets used to it as it starts becoming more and more routine. This is why you hear people complaining about tiredness and such whenever they go past their usual pattern

MiO³

It can stay up longer than 16

CUZ WE GET WORN OUT FROM WORKING.

It can stay up longer. I've stayed up for three days a couple of times when necessary but you need sleep to rest and regain your energy.