I think it's the set of compromises. You'd want a round parachute because the pilot might be unconscious for some or all of the descent - but that makes it harder to control the horizontal velocity.

I'd also think that an ejection being unplanned matters. Skydivers and paratroopers can try to plan their drop zones, while pilots can't. (There's a famous story of an ejection from an SR-71 where the Pilot wasn't sure which of about 5 different states he was in.)

Pilots are probably also a lot less experienced with landings than skydivers would be.