Why did Starfleet need new Romulan evacuation ships?

If Starfleet had to build Romulan evacuation ships from scratch, does that imply it had no carrier-like relief ships?

The planetary scale may justify new construction, but Starfleet should still have existing carriers for smaller evacuations and disasters.

The Romulan evacuation requires a volume of lift far beyond an ordinary relief mission, which can justify building specialized ships from scratch. Starfleet's apparent lack of any intermediate carrier or transport capacity is still strange. Smaller emergencies should already be served by ships able to deploy many shuttles, crews, and supplies. The need for a purpose-built planetary fleet does not eliminate the institutional case for a standing disaster-relief carrier force.