It’s mostly caused by Milankovitch cycles. Which are cyclic variations in our orbit around the sun.

The earth varies in distance to the sun and axial tilt and precession. Like waves, there factors can either overlap and somewhat cancel or they can stack for a larger effect.

The planet is 4.5 billion years old! 400,000 years is just roughly 0.0089% of earth's history (and I rounded up!)

ice ages didn't happen until after the asteroid strike. Antarctica moving into the south pole is likely a large part of it, but they don't really understand ice ages that well TBO. Anyhoo, the Earth was a lot warmer prior to the strike and we have been going through extinction events every 100k years.