One half of a torque converter is attached to the flywheel; why can't the flywheel be lightened to the same effect of a lightweight flywheel/clutch combo on a manual transmission?

 

On a manual transmission the flywheel provides a mounting location for the clutch assembly. When the clutch is applied, the flywheel transfers engine torque to the clutch disc. Because of the flywheel's weight, it can modify power flow from the engine to the transmission.

Torque converter automatics use a flexplate. The flexplate has a similar function to the flywheel on a manual. Many people confuse it with a flywheel from a manual. The flexplate serves as a mounting location for the torque converter. The flexplate transfers engine torque to the torque converter housing. Because of the flexplate's lightweight design it doesn't help smooth out power pulses like the flywheel on a manual. The torque converter provides this function.

Changing the flexplate on an automatic with a torque converter would have almost no effect as opposed to changing the flywheel on a manual can have significant effect.

To modify power flow on a manual you would change the clutch and/or flywheel.

To modify power flow on a torque converter automatic you would make changes to the valvebody and/or torque converter.