As a student I flew in a cloud in a Blanik with my instructor flying, we entered the base of a cumulus at about 1000 m and left at 2100. He was using only a turn an bank indicator (or turn and slip), mine was inoperative because it had no battery.
This part of the flight was uneventful.
This is anecdotal, but as a glider owner I can tell you why my glider doesn't have a transponder:
Monetary cost: A transponder is a substantial investment which requires recurring inspection. Avionics shops tend not to set up shop out in the boondocks where gliders fly.
Energy: a transponder consumes a fair amount of energy and the battery in my plane is ...
Gliders in Europe are not usually equipped with transponders because most of them have FLARM.
As of November 2017, over 35,000 manned aircraft already have a FLARM
system installed, including both the legacy Classic FLARM models and
the PowerFLARM models. Most FLARM installations are in Europe. Over
50% of all GA aircraft in Europe already have FLARM and ...