As someone mentioned it, why exactly don't we routinely carry parachutes in GA aircraft?
As mentioned above. Trying to open the door of your average spam can is almost impossible against the airflow. Gliders have the luxury of a quick release canopy. Bailing from a bulldog would be easy, but a PA28? Jump out the C152 and the first thing that'll happen is you will **** your head on the wing strut.
Gliders tend to flock together and the collision factor is a huge risk. Not such a risk on a gentle sunday PPL bimble. The parachute is a last resort if the glider becomes uncontrollable due to loss of control surface.
SEP cockpits are small enough at the best of times without introducing the added restriction of a large backpack on everyone back.