The verb stem is κάθ-η(σ)-μαι. This is an irregular mi verb with a stem of ησ. You would expect the sigma to effervesce and a compensatory lengthening or contraction to take place, but it acts like an athematic verb, not taking the connecting vowel. "The ς of the verb-stem appears only before -ται, -το. " ε.γ. ἦσται not ἤσεται and ἦμαι not ἦσμαι. See Smyth §789-790 http://artflx.uchicago.edu/cgi-bin/philologic/getobject.pl?c.9:4:43.NewPerseusMonographs
The participle καθήμενος is in the same pattern.