Bicycle sprockets

Why is the profile of a bicycle sprocket different than the profile of a normal sprocket like one in the machinist handbook?
What is the flat at the tooth root for, I have ideas?
Are there any drawings of a bicycle gear? From what I can tell they have about .04" of flat between the pitch profiles which makes the tooth narrower. No wonder they shark tooth so much as they wear.
I get the side profiles for shifting reasons.
I wanna make custom HD Cassette sprockets for high torque applications and could care less about weight or shifting performance.