Senior engineers and scientists, why are you such hard-asses ?

I have noticed this as a general trend, that senior folks work insane hours. Literally life revolves around work and nothing else. You guys even work on your PTOs / holidays.

Is it because at that age you don't really have much to do i.e. children are all grown up, bored with your spouses and not much commitments?

Do you really have that much of upper management pressure or are you just that self-motivated?

Also why are you such hard-asses on your junior colleagues i.e. why do you have unreasonable expectations?