Balance different shipsBecause FDev decided that would be one of the ways they wanted to balance the different ships. Same as they decided it would not be balanced to grant some immunity to interdiction based on size.
It's a game, not a totally realistic simulation. Implementation of supercruise mechanics and interdictions reflect and represent the type of gameplay FDev want to produce, not a full prediction of some future state.
To do this, you need to make them equally achievable (rank of Admiral) to make the same number of slots (for engineering 2 shield amplifiers and 7 is different time in the extraction of materials) and most importantly to make them the same price, because all the equipment on it affects the amount of penalty if you lose the ship !
Here Above wrote, the more effort was spent to buy and modify the ship the better and it can not be compared if these efforts are less !