I think the worry is that, sometimes problems or concepts are too complex to be distilled into a short form. If someone only ever gets the short version of everything, they can lose a tremendous amount of nuance, and the desire for the shortest version may lead people to come away with a misinformed or caricaturized version.

We're already seeing how dangerous that is with how everything has to be a quick soundbyte or people lose interest due to a short attention span, to the point where they ONLY know the soundbyte but feel well informed, when they are in fact still ignorant. This can lead to people being easily manipulated, or coming to harmful conclusions that don't account for enough complexity or variables.

If tiktok and twitter train your mind to have a short attention span, reading long form books trains your mind to be able to hold large concepts in your head all at once, and how they relate to one another. In other words, it is training you to be able to form 'big picture' conceptions about things.

This video on the subject does a good job of delving into the issue.