Powered aeroplane pilots don't as a rule all around in circles under the same bit of cumulus.
Structural failure, when you don't fly into another aircraft, is incredibly rare. Virtually any other emergency is best dealt with by gliding to a field - as you glider chappies obviously do at the end of most flights.
The theoretical risk of engine fire is virtually unheard of, after that most light aircraft accidents start somewhere under 500ft - where a parachute is of pretty much no use.
That said, most aerobatic pilots, do wear parachutes - because they're probably the only people at significant risk of either an unrecoverable spin at high altitude, or a structural failure at high altitude: those two being about the only good reasons to use a 'chute in a powered light aeroplane.
G