Why do NS write back connectors use external ID only and not have an option for Internal ID?

I am looking to use the NS writeback connector to update existing contact records. NONE of the contact records in our system have an external ID, however, there is no option in the NS writeback connector to match internal ID to a dataset column in order to find existing contacts.

Why is this done this way and is there any way around it? We want to pay for/use the feature, but if we can only create new records will be not suffice.