I'm gonna take a guess here - writing by hand is slow enough that your brain has to summarize what is being verbally spoken in order to capture it all. In order to summarize accurately, you need to have some understanding of what is being said, being able to pick out the key points.

I write down all my notes - in fact, got a reMarkable to replace all my paper notebooks - but seems to be the best way for me to retain information. Even though I tend not to reference my notes later.