The whole reason life exists at all in the universe is to propagate
Edit: this isn’t to say that individuals should have to have children or that anyone is living life unfulfilled by not having children. Just that biologically it’s the purpose of life not the existential “purpose” of life
I don’t know if you caught my edit or not, but I was just defining having children as the biological purpose of life rather than the existential purpose. Philosophically the purpose of life is up to the individual.
If you think of it as biological beings and not separating us out as humans, then it is clearer to think about. Like bacteria, we are driven to reproduce, as a species.