The Landlord (and their attorney) have already approved the current lease, so they are comfortable with its terms and conditions. Negotiating a new lease takes time, requires the Landlord to consider new terms, costs money to renegotiate, and is generally a giant inconvenience. It is far more convenient for them to draft a simple assignment on a familiar lease than go through the long process of creating a new one. A large financial incentive is generally required for the Landlord to draw up a lease from scratch.