I learned to type on a manual typewriter, where you had to make a sharp downward stabbing stroke with your finger, then withdraw it quickly with a snap of the wrist and recoil.
Striking the key hard enough to make the levers move quickly and smoothly was not easy, and I suspect that the spherical shape was intended as a sort of "socket" which held the fingertip securely in place. In particular, the rise at the back of the key was probably intended to prevent the finger from moving away from the typist.
Since we only "press" keys now, instead of actually "striking" them, the forces involved are far lower.
Why cylindrical vertically instead of horizontally? Just try to imagine it!