Competition kills margins (profits, security, QoL), so the budget for eradication should be quite high, but generally speaking the idea is to destroy even fledgling upstarts, back when the cost is low.
You might or might not fatally cripple the opponent, but retaliation can do that too and you cannot be sure that it won't. It's MAD all over again.
In those terms, the US should have been nuking and dominating everyone, and the idea was floated after WW2, but I believe they were precluded by practical limitations.
If they had developed the tech outside of wartime, and built up a stockpile, maybe that is indeed what would have happened and we'd have a one-world government already.
You wouldn't be able to know this over the vast distances of the universe.
People are arguing two contradictory things: these are unfathomable alien civilizations with motivation and timescale we cannot comprehend, but we would perfectly understand their tech level, location, and capability of striking back.
It doesn't add up. It's a scifi premise needed for interesting plot conflict.
It relied on two principles "the chain of suspicion" and "technological explosion", which don't hold true if we are on the same planet. You can google it (or llm it) :)