What makes reading books better for your brain compared to reading posts on social platforms?

I do think they are different, especially in terms of how they affect your attention span but I can't quite articulate why. Is it because on twitter for example, you're going from topic to topic without focusing on one singular thing? What about if you're reading down a reddit thread and everything you read is about the same topic, is that the same as other sns activity or better? Is it about the complexity or content of the writing? Because I read a lot of insightful things online. Shouldn't it not have much difference on how your brain is working compared to reading books?

I can feel the negative impact of my phone on my brain, and everyone says read more books but why exactly does that help whereas social media consumption is so bad?