King Janaka and others (Knower kings) have attained spiritual perfection by performing activity alone. Seeing the need for lokasangraha (societal welfare) also, you deserve to be active.
Krishna was not the first exponent of spiritual wisdom. He cites Kathopanishad in 2nd chapter, while describing the imperishable Self (2.19,20). He has quoted Seers of Truth (2.16) also. Here he refers to King Janaka, Seeta’s foster father. As a good Teacher, Krishna only re-states the ancient wisdom he had learnt from others.
To inspire Arjuna and lead him to activity, Krishna brings the illustrious ruler Janaka, an acclaimed Knower. Seated on the throne, Janaka acquired Self-knowledge and also imparted it well. Sage Vyasadeva, finding his son Shuka unresponsive, had sent him to Janaka to get the real spiritual instruction.
Krishna says Janaka sat on the throne, even as a full Knower. Arjuna should follow this example, and take to his activity, however harsh it is, without the least doubt, dislike or resistance. In that lies Arjuna’s harmony and fulfilment.