Why does sending tanks improve rolls?

It is evident to everyone that using infantry is more cost efficient than tanks. Nostrat tanks have the cost of 2 inf, but only double the attack and two thirds the defence of one infantry.

However, whenever a 100% inf rush fails, it is stated that it failed because there are "no tanks". Also, when any rush or attack is done, tanks are recommended to be present since they 'improve rolls'. Noting that stacking bonus is calculated partly on unit type, why does this occur, instead of tanks just getting raped by a massive stacking bonus and dealing little to no damage at all?

Because when you attack, you attack and defend the same time.So it is better to have a mixed stack(depending on the situation send more or less tank than infatry), which can have good attack and good defense at the same time.