Because a converter like that is expensive and is extremely inefficient outside of the very narrow rpm band it's designed to operate within. Go spectate at any Top Stock or Super Stock event. Some of those guys use only 500rpm going down the track.

 

Yes, they use narrow bands and tight ratio gears....stuff our cars do not have so you have to maximize power in the areas between gear drops. This is a Much wider window than pro stock and such. My point is you can have similar power numbers at peak rpms between 2 combination of parts or two different cams for that matter, but between rpm drops between gear changes there can be differences in HP and TQ in the curve. The higher average power will win out.