For sure, what you're saying is true, but I suppose it depends on the content, layout, and how it's used. Vertical is typically better when first word type scanning is important, like data tables or anything similar. Horizontal scrolling in data tables is something I hate with a fiery passion and do everything I can to avoid, for example. When a workflow or series of interconnected elements are what's important than horizontal tends to work better for how people think about those things, imo, but there are exceptions to every rule. Basically, it just depends.
And phones are portrait and web browsers all naturally want to scroll up and down.
The Git graph tools are vertical because terminals naturally scroll up and down.
So all my hand-drawn graphs are vertical, too. Top to bottom, just like code or very narrow prose.