What causes decline in player performance?

[*ochocincos*]

Just a few years ago, we were all excited for Jonathan India and Tyler Stephenson.

Stephenson 2021-2022 was batting ~.300 and OPS ~.800-.850
However, this season he's down to .251 BA and .701 OPS.

India is similar:
2021 (rookie year) India had a .269 BA and .835 OPS. Won ROY.
Since then, he's batted ~.250 and under .750 OPS.

We see this happen with players all the time, especially younger players.

Since I did not grow up playing baseball, I am curious as to how players end up declining as they get more experience in the majors.
Is it because enough tape is now out there where pitchers/hitters are learning how to attack them?
Is it they are trying new things with hitting/pitching but it's just not working out as expected?

This is my frustration with counting on all these young guys and not making trades for proven veterans sometimes, as these good rookie seasons might not end up being reproduced in the next 2-3 years when all is said and done.