Some farms need to be connected to other buildings, some don't. Cider farms for example don't need any other building other than a warehouse because they produce a final product, but hemp farms produce an item that makes another item, therefore hemp farms need a rope and/or linen maker as well as a warehouse in their range.
You don't need the rope / linen maker in range of the hemp farms, they don't even need to be on the same island aslong as you ship the hemp.