The thing is, Arguin to Marrakesh really isn't that far away... it's shorter than some trade routes we see, even give deep sea "distance compression".

Which makes me think that the trade routes aren't "generated", instead they are pre-defined somewhere. That, or like I said, they are disallowed from crossing deep sea. But really, London, Lisbon, and a few other places absolutely should be able to hook in to new world ports.