Because their killer robots killed them all (the Berserker Hypothesis):
Rather than having a single proponent, the Berseker Hypothesis is one that appears to have emerged over time, both as a popular science fiction trope and as a potential resolution to the Fermi Paradox. In the case of the former, science fiction writers like Fred Saberhagen explored this idea at length with his Berserker novels (1963-2005), the popular SF series from which the theory gets its name…
There have been a number of variations on this argument, such as cosmologist Edward Harrison. In 1981, he argued that an advanced species that has overcome its own self-destructive tendencies might be motivated to create Berserker probes out of a sense of self-preservation. This idea, where destruction is implemented out of some sense of “the greater good,” has come to be known as the “Cosmic Quarantine Hypothesis.”
There’s also the version of this hypothesis where the first lifeform (or lifeforms) to achieve interstellar space travel and colonization will prevent others from arising and achieving the same ends. This may be intentional, or simply the result of the “tragedy of the commons” and the “anthropic principle”, where one party working for their own self-interests invariably holds back others.
Matt Williams, “Beyond “Fermi’s Paradox” VI: What is the Berserker Hypothesis?” at Universe Today
And so forth. Williams offers the common sense rejoinder that, if robots killed them all, we should now be dealing with the robots. In some quarters, common sense is doubtless considered to be overrated.
See also: Tales of an invented god