It should be clear that King against King, with no other pieces, is a draw. Neither side can check or give checkmate.
The diagram is King and Knight against a lone King. To announce checkmate, White must give check plus control the three squares around the cornered black King. The white King can control only two squares, while the Knight is incapable of checking and controlling the last square at the same time.
The same logic applies to King and Bishop against a lone King. The game is a draw.