What causes a player's aim to decline?

It's not like real sports where you lose your fitness, no? How come players will fall off in mechanics within a year?

mental
play with nothing to lose vs everything to lose

Old age
Reaction time gets slowed over time from what I know. Don't ask how.
Also mental and confidence plays a huge part

they dont, level just gets higher, experienced players are still the same but the level of the new talent is higher so they are no longer as good, easy to standout 1-2 years ago but the level is way higher now

they dont train their aim, here is an example : https://youtu.be/a0aCVn7FEug?si=YofgFC5zFkFH3H4L