How come your ratings go down when you don’t play?!

I could be wrong, but I think because the averages go up when the number of games increases.

I believe there is a competition component to the ratings. Not sure though of course.

But basically as other players play more and the squads play more, it will affect you, even if you miss the match(es) in question. This is because your previous opponents might drop in rating themselves, meaning your previous matches don't look as impressive.

Maybe anyways.

I think that smartly incentivizes players to be active and attempt to compete at a high level throughout the season should they wish to remain at or near the top of the ratings leaderboard.

Or, to look at it another way, it gives credence to the notion that an above-average player that plays in most or all of their squad's matches provides more value than a one-and-done performer with a particularly strong showing.