We are still in an ice age and we just happen to live in an interglacial period. The Earth has the large polar continent, and multiple other continents arranged

We are still in an ice age and we just happen to live in an interglacial period. The Earth has the large polar continent, and multiple other continents arranged such that they greatly impede the free flow of water currents around the globe that are necessary for mass glaciation. In these conditions, glaciation can easily lead to a positive feedback loop and once that gets going the effect is rapid (think order months to years, not decades).