That's odd. At my local pool they very definitely are in the water with us more often than not during lessons. How else can they demonstrate techniques, and watch us closely to see what we are doing wrong?
There is one exception I've noted - the coaches of swim teams seem to mostly stand at the front or back of the section their team is using. I'm not sure what that's about, since I've never been on a team like that. But I can guess it has to do with seeing everyone at once.