Perhaps the life forms with long-term success have learned that it is not prudent to broadcast your location; the smaller the chance of other civilisations knowing about your existence the better.
https://en.wikipedia.org/wiki/The_Forge_of_God
and
https://en.wikipedia.org/wiki/Anvil_of_Stars
Anvil of Stars is often said to be the weaker of the pair, but it haunted me for several reasons, not least the extremely spooky ending!
In the books game theory means that the most prudent way to deal with a competitor species is to obliterate it immediately on discovery. The answer to Fermi’s Paradox is that everyone is keeping their heads down and and hoping no one notices them.
Humans, on the other hand, are merrily broadcasting away without a care in the universe...
Your point doesn’t really hold up if the effort to destroy a civ is basically zero, while the cost to defend is approximately unachievable.
For all we know, the universe is jam packed full of Predator-like aliens who are the dominant species on their planet specifically because their society is built around dominating everything they can.