Well, it's true that the medium has been saturated, and innovative and evocative gems are rarer. But, it's also a case of you getting older.
Humans experience motivational salience primarily through novelty. If you've watched a lot of anime/read a lot of manga or LN, you tend to internalize and compartmentalize all the tropes, narrative devices, and themes. Very little is novel anymore, especially in a medium that's already very formulaic. That predictability and absence of novelty literally translates to your neurons failing to reach threshold potentials, via the D1 dopaminergic Incentive salience system, and your limbic system refusing to engage.
A medium has primary positive reinforcers, that make it objectively good, like beautiful cinematography, art, music, prose, narrative, et cetera.
A medium also has conditional positive reinforcers, that make it subjectively good, which can be its theme, its characters, tropes, literary devices, setting, et cetera.
Once you've watched enough anime/read enough manga, it gets progressively harder to get emotionally invested and/or intellectually stimulated, because you've already experienced approximations already, dozens if not hundreds of times. The stimuli are simply not strong enough anymore, you've built a tolerance.
Imagine reading an amazing book. It can make you cry, ponder, laugh, daydream, and even change your outlook on life. It can be an intense experience. Now, imagine reading it a second time, in quick succession. Will it evoke the same emotions? Will it have the same intensity?