I think Authors is way way optimistic with these numbers: "Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life" and

I think Authors is way way optimistic with these numbers: "Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life" and " "And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth" Why 1% and not 10^-10%?