Originally Posted by
SoSmellyAir
The fork potentially affects both reach and stack because a longer or shorter fork effectively rotates the frame around the BB, moving the top of the head tube relative to the ground.
Yep. Without the fork, you wouldn't be able to identify horizontal or vertical lines through the BB, nor seat tube angle, nor head tube angle. (Even with a fork, you'd get different horizontal and vertical lines if you put two different sized tires/wheels on front and rear). Frame stack and reach (and STA, and HTA) depend on things that aren't part of the frame; parts of the frame that affect fit don't depend on stack and reach. Dan Empfield agreed when I pointed that out. Other frame builders have also agreed.
That stack and reach work as well as they do considering these points is actually kinda nice.