it's default because it is analogous to something else

money is a bit like fat, you put it on in summer so you have it to burn in winter, many animals do this... think of squirrels, they are literally a verb for saving in english... how did that get to be that way? yes, because they collect stuff so when teh supply is short they don't run out

rapidly decaying fat would not be stacked away by people

even sugar doesn't decay, in fact sugar decays slower than fat, so if it's about carbon based food energy you'd see ... idk... grain silos and stuff...

default behaviour for a reason, they just switched out the thing

no more tallow, now it's nasty overheated rapeseed oil to rape your winters