I have always wondered about this one.
Pulling high G-forces is, I assume, the most physical challenge the pilot of a high performance fighter aircraft needs to be able to sustain without blacking out or worse. Indeed, in most countries in the world would-be fighter pilots cannot qualify for jet-jockey status without first passing the dreaded centrifuge test.
Further, modern jets can in fact pull much higher G-forces than the pilot without damage to the aircraft. Indeed, I read somewhere once that one of the issues with auto-missile-avoidance systems is that the aircraft is limited to performing maneuvers that the pilot can withstand.
As I understand it, the issue with high-G turns is that the blood is drained or driven from the body to the lower (and upper when undergoing high negative-G) extremities of the body resulting in starvation of the brain and consequent blackouts or even embolism in the negative-G case.
I realize pilots wear tight G-suits which have air pumped into them to restrict the amount of blood that can accumulate in their lower extremities, but that doesn't reduce the stress on the heart trying to pump blood up to the head.
It seams rather obvious to me that much of the issue with high-G is caused by the seated position of the pilot. It is an established fact that the more vertical you are the harder it is to withstand G-forces.
Why then are modern jets not designed with the pilot in a more reclined position? For example, the position used by Formula-1 race car drivers:
Don't get me wrong, I understand F-1 drivers do not need to withstand the same kind of sustained vertical forces that a fighter pilot does, however the legs-up reclined position seems to be far more practical for a high-G environment.
I do realize that the whole ejector-seat mechanism would need to be redesigned to eject the pilot differently, and that the pilot's ability to see behind himself would be compromised without technical aids, but surely the increased turn rates and lower profile cockpit would outweigh those issues.
NOTE: I have seen other posts on using the head first "prone" position, including this stack-exchange link but nothing on reclined.
ADDITION: Found this image of a Foka-5 glider pilot position too... note how wonderfully streamlined it is.