Size of our galaxy is about 100,000 light years, means we are looking at most 100,000 years in the past if we are only interested in 100+ billion stars in our galaxy. 100,000 years is short time relative to age of stars or planets.

Read https://en.m.wikipedia.org/wiki/Drake_equation for some estimations on how many civilization we should see.

Also relevant: https://en.m.wikipedia.org/wiki/Fermi_paradox