We know that our knowledge is there, somewhere in the brain, but it doesn't work.
I've seen me and my opponents at 200-300 elo below our maximum after paused for less than a year.
The answer "it's like when you don't play football for a while, the capacity of your heart to supply oxygen for a long time worse, and your muscles became smaller" is not an answer, because the brain is not a muscle.
"It's like when you don't play piano for a while" is also not an answer, because then the next question would be "why when you not play piano for a while your play get worse?".
me write sentance in long time make worse my speaky speaky.
I'd imagine for similar reasons that people forget languages, math, and etc when they don't use them. You probably play a bit worse because you've forgotten certain patterns. Humans simply don't have perfect memory. In this case tactics and strategic motifs depend on pattern recognition. So I'd conjecture that it's simply a manner of forgetting some of the nuances of your better play.
Why do we forget anything at all? Why do we forget words, faces, names? What did I do 15 years ago on March the 2nd at 5pm? I don't know!
It's not ideal but that's how it is. Maybe some sort of optimisation where we're best at skills needed recently... although ideally why not remember everything all the time? Maybe we can answer this after we understand the brain better.
Also, this is a better question for a medical type of forum... but maybe someone here has a good answer.