I opened up the hood of a car once and there was loads of empty space inside there. Why are they not filling up every cubic inch of the engine compartment with

I opened up the hood of a car once and there was loads of empty space inside there. Why are they not filling up every cubic inch of the engine compartment with more pistons, or making the compartment water-tight and then filling it with even more petrol to increase the car’s range? Admittedly I don’t know a single thing about car design, but surely all of that empty space is an indicator of poor design right?