Because not everyone has 1000+ hours to sink into designing fleets of ships? This game is a huge time commitment if you want to produce truly decent craft, and death cubes give you a lot of bang for your buck. PMWs take a lot of work to get right compared to arrays of gats. The investment in design time should have better payoffs than it does. Of course production costs and welding logistics in survival are huge, but not everyone even plays mine and weld-it survival. Lots of people play "copy and paste into survival and then have a fight" etc. You can still balance around material costs and ammo supply but actual construction isn't an essential part of combat and so it isn't the deciding metric for combat balance. Cost is. Also, not every death cube has to be a literal cube with armor and guns on every side.