Our calendar marks days and dates important to our existence but they all start from the Exodus from Mitzrayim which makes sense because
A -- that's when we started fixing a calendar that related to us as a people
B -- that's when we started identifying as a discrete nation
But in the Rosh Hashana davening, we connect R"H with the creation of the world (or some aspect of it) by saying "Hayom Harat Olam" so we acknowledge that even prior to the Exodus, there were important moments in our history. And yet, there is little else that we do to mark the dawn of our existence on this planet.
The creation of the world (and of mankind, which we seem to commemorate in the kiddush for Shabbat, and through the idea of Shabbat) and the loss of almost all humanity, and the reestablishing of the human line both seem to be rather important steps towards the formation of us as a people. The Chumash takes pains to establish the lineage of us and explain how we came to be instead of starting with Yetziat Mitzrayim. How come these two moments in history are not attached to specific celebrations (or memorializations) in our calendar? The deaths of the Mitzrim at Yam Suf affects our saying a full hallel, but the death of 99% of humanity has no impact on our prayers?