I think that whats missing here is consideration of the fact that the USF can equip every infantry unit they have with a zook, or even double zooks.  A 4-rifles blob firing 8 zooks every few seconds is bad enough as it is, and a dire threat to all axis armour.
Ost can't field nearly as many PG's, and OKW can only upgrade one shreck.  This is quality versus quantity again and making zooks identical to shrecks would almost certainly destroy any parity between the two sides completely.
ETA: and making zooks cheaper would only make it even easier for USF to mass blob zooks.
A dire threat to ALL Axis armor? Except the most common and most important late game tanks you mean, like the Panther and Tiger. 
I agree with the fact that Zooks are spammable and they would be abused if they were price buffed or damage/pen buffed, but they most certainly are not effective against arguably the most important Axis armor. 
While Shrecks are effective against every USF tank from any angle.