age and hormones.

testosterone seems to be involved because it is mostly an issue for men, not so much women. maybe also why women live longer.

but there may be other hormones involved - perhaps cortisol ? ( it's a stress hormone known to make people get out of shape )

also since it's called "beer belly" perhaps alcohol has an effect too - perhaps directly - perhaps indirectly by damaging the liver.

i haven't looked into it. i am sure if you google it the information has to be out there. they wouldn't try to hide this because they know this is way deeper than the average person would ever look.

they only hide the stuff that is obvious because they know people's attention span.

i haven't looked because it probably wouldn't change anything - alcohol, cortisol and calories would still be bad for you - and i still wouldn't give up testosterone even if it killed me.

but if you're curious you should absolutely research it and let us know what you find.