re: Right now I feel like I could take on the whole Empire myself.

An actual in universe answer would be that adding an extra laser and the apparatus to have it rotate would reduce speed/mobility and increase size so attack surface. To add those components to a ship would need a power core like you'd see on the Millennium Falcon or Outrider or Slave 1. X-Wings and A-wings were built for fast attack and dodging enemy fire.