Any civilization building simple space technology the aliens encounter might keep being harmless to them, or given some thousands of years to develop in peace it might both develop robust interstellar flight and go all genocidal on everything in the stellar neighborhood. If you destroy the weak alien technological civilizations you encounter right when you encounter them, you make sure that they won't be coming after you in the very long future you expect your civilization to keep sticking around. And assuming you are already thinking like this, you might also be more likely to think clever aliens will also think like this and be even more disposed to not let them stick around to develop to a level where they can actually pose a threat to you.

It also helps that if you still live on planets, anyone in space can lob really big rocks at the planet with little danger of retaliation to produce handy extinction events, so the game theory is more first strike wins than mutually assured destruction.