Are there any reasons why junction-to-case (or channel-to-case) thermal resistance is not specified for small-package devices?

This is because heat dissipation mainly occurs through the leads, and the package surface area is very small, making it difficult to accurately define and measure a representative case temperature.
For this reason, junction-to-case or channel-to-case thermal resistance is not an appropriate parameter for thermal evaluation of small-package devices.

For information on calculating junction or channel temperature, please refer to the following FAQ:
> How to calculate the junction temperature of a semiconductor device whose datasheets do not include a “thermal resistance” value