Having just the notary there relies on trusting the notary to be an impartial party, with just one witness in addition to the two parties there's only one point

Having just the notary there relies on trusting the notary to be an impartial party, with just one witness in addition to the two parties there's only one point of verification meaning only one person you have to corrupt to be able to lie about what happened. Bringing in more people increases the size of the conspiracy required to lie about the transaction.