Quotes by Sersi

Sersi: "We're Eternals. We came here 7000 years ago to protect humans from the Deviants."
Dane Whitman: "Why didn't you guys help fight Thanos? Or any war? Or all the other terrible things throughout history?"
Sersi: "We were instructed not to interfere in any human conflicts unless Deviants were involved."

It's one thing for humans to never learn that - their lifespans are short, you turn your head for just a moment, and an entire generation has gone by.