i think the only exception to this is if the fiction in question is supposed to be realistic
DARPA has a real life ongoing project with the goal of building an autonomous robot that can "eat" plant matter in order to keep itself powered. It is not nearly as far-fetched as you think it is, it's just that it's rarely ever depicted in fiction.
but it is though. that fried chicken isn't going to give nearly as much energy as the same amount of actual fuel would
You're right, being able to adapt and utilize different sources of fuel is completely ridiculous. It's why us humans only eat one specific food and ignore the rest because pfft why would we eat
that if it doesn't give us as much energy as
this.