system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it when I play a game of chess after not playing for years, I'm nowhere near as good as I used to be? What exactly has my brain forgotten?

I still know all of the moves of course, and I'm sure I still have the power to read moves ahead, how exactly has my brain changed so that it's not as familiar anymore?
gpt
Chess is a game of pattern recognition. There are thousands of motifs and patterns, so many that even our super grandmasters cannot conciously sort through them all. Your nervous system or intuition helps you make choices. The less exposure your nervous system has, the less likely you will be able to make/find the correct moves or avoid bad situations.

You remember the moves, you forget the tactics. You remember the moves, you forget the positional strategies or themes. 
Also, age and drug use can both limit the clarity and accuracy of your calculations.