I observed it myself with a lot of things, esp. when it comes to learning an instrument over the course of almost 2 decades (piano specifically, in my scenario). Sure, you can pick a few notesheets and keep practicing the entirety of them from start to finish at full speed over and over. However, the process will be extremely non-efficient and will make you take much longer to learn the song, and it won't make you take away as many lessons from it that are unrelated to that song specifically (i.e., those "unrelated" things that make you a better piano player overall).
Instead, you need to be mindful of what specifically goes wrong, what goes right, and target practice those problem points specifically. Let's say you have a few bars with a syncopated rhythm in a song you are learning, and you are struggling with that section specifically, you nailed the entirety of the song otherwise. Instead of just keeping playing that entire song over and over, you should practice just those few syncopated bars by themselves at a very very slow pace with a metronome. It will feel very awkward at first and give you a feeling that you aren't progressing much. Then you start incorporating that small problematic section into the entirety of the song. Then you speed up the tempo on that specific section and practice it at that tempo over and over. If the problem is in your left hand, you practice the left hand by itself first, then graduate to both hands. Then you try to incorporate it at a higher tempo into the rest of the song.
The whole process of that sounds very tedious and painful, but mostly because it actually is. However, it will lead to much better results. Not only you will learn this specific piece much quicker using such a methodical approach, you will perform it much better and way more consistently at the end. You end up dissecting that problematic point so much, whenever you see syncopated bars in the future in completely different music pieces, you will have much less problem with them. Which will allow you to spend more time improving other aspects of your play or learn other techniques. All of that learned knowledge ends up snowballing and compounding so much, over the course of a few years you end up massively outperforming someone who just blindly kept practicing pieces over and over from start to finish until they got them right.
