They're not destroying planets though... they turned 1 into a machine. Considering the rate at which they did this, and how long they have been around, they could have mechanized ever planet in the solar system before we ever left our own atmosphere. Maybe mercury was chosen strategically because it couldn't be terraformed like the others (perhaps due to extreme temperatures/conditiond). They were on venus for billions of years and never mechanized that planet. Honestly, what better way to fight a darkness that destroys all light that it touches than with an endless army with no light of their own?