If we were to have a mass extinction event today, the "survivors" would quickly find that all the readily accessible surface level fuel sources are long gone and have already been consumed. In the 1600's or 1700's it wouldn't have been that difficult, but today the challenge of getting access to power is going to make restarting civilization somewhere between difficult and impossible.

Try finding enough burnable wood near an emerging city/town to provide warmth and power while you restart civilization and I think you'll discover that humanity has already cut down the majority of the forests, tapped the majority of the oil Wells that can produce oil without lots of supporting tech, and mined most of the easily mineable coal. Sure, you can get a bit of power, but it's going to take a lot of both power and specialized skilled labor to restart our power sources civilization after a mass extinction event. The chances are extremely good that even if you can resume making babies you can't resume making the power those babies will need to achieve the goals we would have for them. On the surface of the Earth today it takes more power and skills to gain access to power than survivors are likely to have.