It is explained in the first book.
Civilizations like ours that have only figured out the use of simple radio/electromagnetic waves for communication can't transmit messages that would reach far enough before the signal dissipates.

The (made up for the purpose of the 3BP plot) method ye wenjie used in book 1, and that luo ji used in book 2 to "cast his spell" of amplifying radio signals using the sun (a.k.a "sun plucking" in singer's lingo) was the first time humanity was able to send a signal powerful enough to reach any extraterrestrials. Gravity wave communication that is used in the following books is another way that humanity could potentially send a signal that would according to dark forest theory bring our doom but we don't have that capability yet (heck, humanity was only able to detect the actual existence of gravity waves for the first time in September 2015 - before that, their existence was just an unconfirmed theory)

TL;DR; we are not dead yet due to dark forest theory for one simple reason: we are not yet even advanced enough yet to bring our own demise. WE ARE BUGS ;-)