Why are 3- or 4-seat gliders so rare?
Because much of the fun in being in a sailplane, is in actually flying the sailplane.  It's not a means to get from point A to point B, it's a means to enjoy the magic of flight, much of which involves seeing the sailplane respond to your control inputs.
Once you start getting more than two occupants, most of the occupants won't get to do much of the flying.
It's kind of like asking, &quot;why aren't there more saddles for horses that can handle 3 or 4 people?&quot;
As an aside, raw novice passengers tend to be at high risk for airsickness when subjected to prolonged thermal turns in turbulent conditions, especially if they get to do little or none of the actual flying.  Three or four non-flying passenger seats would potentially equate to a lot of clean-up afterwards.
I guess it's all a little subjective-- a commercial pilot friend of mine regularly gives 15-to-60 minute semi-aerobatic rides in a glider with room for two passengers squeezed in side-by-side on a seat in the back (Schweizer 2-32), with controls for the pilot only, and most of his passengers love it (and most don't get sick).  But being seated tucked in close to a friend for a little while feels different than riding on a &quot;bus&quot; with, say, four or more strangers-- and in that particular case the very same glider can have the second control stick re-installed in the back and be used for student instruction with one rear-seat occupant and one front-seat occupant.  A bigger &quot;bus&quot; with more ample seating for multiple passengers probably would be a poor candidate for a training glider.