I think this is a byproduct of devs overdoing it as usual to make sure we notice a new feature. (hammering, more like)
My unmodded Del Sol Valley save had leaf piles spawned on day 1 of fall, no tree was losing leaves at that point, and yet.
So many cool features in theory end up being plain annoying in the game because they happen too much / all the time.
It's like for the devs, no one is playing their life simulation more than 5 minutes at a time. Leaf piles have to be spawned right away everywhere so people notice the new gameplay, and obviously no one will play long enough for them to decay, don't be absurd.