There are technical limitations.
When I fly a two-seater, my thermalling technique is different from flying 15m single seaters. With the single seater it is no problem to center the thermal while observing the variations in climb speed. Tighten the turn when climb speed drops and vice versa.
But with two-seaters I wait for a three quarter turn and only then apply my correction. That means I have to wait with a correction and have fewer chances for them. If I wanted to fly like with a single seater, my corrections would come too late and shift me further from the center of the thermal. Two seaters are heavier and have more wingspan, so both inertial moments and roll damping are greater and make all responses more sluggish, requiring me to fly corrections more deliberately.
Now extrapolate that to a three- or fourseater! While thermalling a single seater (Discus, ASW-24) is fun, doing so in a large two seater (ASH-25 or SB-10) is real work. I cannot imagine doing this successfully in a glider that is again a lot heavier.
The only gliders with more than two seats were designed for gliding only.
More detailed explanation:
After two or three turns in a thermal you have already a good mental picture where you are relative the core of the thermal. A single seater is agile enough to make corrections immediately, using the change in climb speed as a cue.
A two seater (and an open class glider) is more sluggish and requires to think more ahead. Now those immediate changes become counter-productive because they will be out of phase with what you intend. You have to wait your turn (literally) for the next change, so you will be slower to optimize the flight path.
On top of that: On weak and especially cloudless days thermals are narrow and the higher wing loading of the two seater forces you to fly wider circles, so you will stay in a weaker thermal than the single seater which can fly tighter circles. Going to more seats will drive up wing loading and inertias more and force you to fly even larger circles which give you even less climb speed.