The reason why a 1 or 2 point missile or torp can't roll 4-5 attack dice is because the value of each subsequent attack value is non linear. An increase from 3 to 4 is more valuable than an increase from 2 to 3, or indeed from 1 to 2.
This is why a single Phantom has a more powerful attack than two TIE/ln-s'. The 4 dice attack is stronger than two 2 dice attacks, and it is priced accordingly.