"They are unable to respond. They are unwilling to respond."

In addition to the fact that the Milky Way galaxy is really, really, really
big and has lots and lots and lots of stars - my favorite example is
comparing stars to salt grains. A handful of salt contains about 10,000
grains. To equal the amount of stars in our galaxy, you would need
enough to fill an average school room to the ceiling and have the
salt/stars pouring out the windows - my other thoughts on the
matter are that if ETI aren't advanced enough for even radio and/or
the concept of life elsewhere, we aren't going to find them or they
us any time soon.

As for the really advanced ones, they probably have little reason
to communicate. Study us, maybe, but then you don't disturb your
subjects of study.

They are also likely to be Artilects, which means that while they
may not necessarily have a "ban" on mere organic life forms roaming
the stars, they probably have as much in common with us as we
do with ants and react in about the same way as we do to those
insects. Plus as I have brought up before, they also probably
dwell in the very outer reaches of the galaxy, where it is so much
cooler - by which I mean temperature-wise.

But my first thought on why we haven't found ETI is "simply" that
the galaxy is huge in size and numbers and we haven't gone far
enough into it to warrant any serious notice. Even our radio and
television signals have barely penetrated the galaxy and most of
them are faint.