When humans and aliens spotted each other, aliens sent forward a scouting party, whereas humans retreated with a scorched earth policy.

Adam and Eve were reborn and raised to deal with the war situation but they were just children barely coping with psychological issues. They grew up with too much time to wait but too little energy to waste.

Loneliness, lack of purpose and mental health were indeed linked to casualties: Gabriel the first reborn attempt, Jeremy their adopted son and incidentally Jonas their naturally-born baby.

At their back was their home, now clearly intending to destroy them. In front was an approaching alien fleet.

They decided to live on, i.e. to flee forward into the unknown.