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

Neither junction-to-case thermal resistance, R_th(j-c), nor channel-to-case thermal resistance, R_th(ch-c), is specified because small-package devices do not dissipate much heat and because temperatures on the surface of small packages cannot be measured accurately. For thermal design, use either junction-to-ambient thermal resistance, R_th(j-a), or channel-to-ambient thermal resistance, R_th(ch-a), instead.