I just thought of something.  I said that there weren't any demigods around to help the gods conquer the planet, but technically there are demigods on Earth.  They are just the descendants of the gods that got left behind or stayed here on purpose (like the rebels that sealed the Foci), so it's highly unlikely that they'd help.

Most of them wouldn't have had any powers before the Foci were unsealed either, as the only two ways to catalyze the formation of an Essence crystal in the chest is to absorb a monster's or, for a god, demigod, or monster, to absorb strong human emotion.  The Foci absorb the emotional energy of millions, especially faith, and feed it to anyone that can absorb it in the area.

Without a Focus they have to absorb it by being in very close proximity when someone experiences that emotion, and can only recharge their Essence crystal by absorbing more.  For this reason many gods that got left behind started cults so that they could absorb energy from a few devout followers.