[squeaky-clean]

Mutually assured destruction on a planetary scale does come about rather early in a civilization's history (if we're assuming galactic, or beyond-galactic scales of civilization). But that doesn't really mean it is easy to come about on a galactic or universal scale. Is an enemy destroying a planet or solar system you control really such a big deal when you're battling to control a third of the universe? Even having an entire galaxy destroyed would be like an enemy destroying someone's living room on an earth-scale.

Of course, this is all just making guesses about technology way beyond our current comprehension. Maybe universe-destroying bombs come up rather early in technology on this scale. Who knows.