You're right that some things are more subject to control than others. We can 'hold it' (whether breath, bladder or bowel) - but only for so long. And, with years of discipline some people (yogis, frex) can learn even greater control. It's definitely a good thing to be able to hold your breath when you're in the water, and to hold other things when you're in adverse circumstances.

Maybe there's some evolutionary advantage in learning to control our very metabolisms. But, given that this is not presently how things work, it seems weird that so many people seem to expect it.