Refurbished device availability is always in flux. Also, when a new model comes out, sales of the most recent discontinued but now available as refurbished devices also goes way up. So combine limited and variable availability with increased demand and sales, and yes typically refurbs go very quickly and the supply dries up.
Also note that Apple keeps some refurbished devices in their stocks to have service units available for those models when customers need them replaced under warranty or are paying for an out of warranty replacement. In the USA and other countries, companies do have legal requirements to keep reasonable supplies of replacement devices in stock for the lifetime of the device, even after it is no longer being made or sold as new devices.