system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does it take us weeks (even months) to feel fit, but only a week of not exercising to feel like we're back at square one?
gpt
Our body is a multi-million year old machine that has evolved into being able to adapt to changing environments as we move around the planet and hasn't quite picked up on the massive changes to modern living the past century. You're emigrating, cool, don't need those upper body muscles just need strong legs and feet for moving. You're farming? Cool, need some strong arms to use tools.

Your body gets stronger by muscles tearing and rebuilding stronger. You go and squat 100lbs 10 times four separate times in 10 minutes, your muscles tear. You do that a few times and your subconscious says to itself "Hey, we apparently squat 100lbs 10 times four separate times now. That's what the conscious is doing now so lets rebuild the muscles to be able to do that."

You keep doing that and your body maintains it at that level. You add more weight your body responds by going to a higher level. You STOP doing that your subconscious quickly says "Hey, we apparently aren't squatting 100lbs anymore so redirect those resources to storage because we don't know the next time we'll get food." 

TL;DR Muscle takes quite a bit more calories to maintain than fat does so if you aren't using it your body believes it is doing itself a favor by getting rid of it.