I personally think that its not as much as an issue as thicker notebooks creating an uneven writing surface.

Being right handed, your hand is supported at the same level as the writing surface until the very end of a line, where you typically leave more space.

Being left handed, you start every line of writing without your hand being on the same surface as the writing surface, which especially sucks if you have issues with handwriting (which I annecdotedly notice is more common in lefties).