Q: "Why do all glider pilots wear a parachute" ?
1 - Because they can
2 - Because they're supposed to - In the UK, the BGA Recommended Practice is that occupants of gliders operating from BGA sites should wear a servicable parachute "subject to the glider being fitted to accept the occupant(s) wearing parachutes".
My guess is the question could be turned around to "why don't powered aircraft pilots wear a parachute?". Typical answers being insufficient space in cockpit, aircraft not designed to cater for it, etc. Quite similar (& valid) answers in fact to "why don't gliders carry transponders?"
If you drive a car would you prefer it to be fitted with an airbag or not ? So, if your aircraft design would permit you to wear a 'chute wouldn't it make sense to ?