As I said: They are not set in stone, and yes they were drawn fairly arbitrarily. However, they WERE drawn on geography, for example often at narrow parts of a sea, or at a cape or hook.
One thing I intentionally ignored were existing sea routes. I don't see a reason to take them into account, because they were just as arbitrary. So the argument "this makes an existing route longer" holds no water to me, because at the same time it will make other (not-yet-existing) routes shorter, or even possible.
Also, the yellow zig-zag lines are not entirely true, and thus misleading. The actual routes are a bit more tricky, but shouldn't really matter. Judge travel times, not paths that don't have any in-game meaning.
