God's Plan.
"Alters are divine conduits. If these gods want to reach this planet, their worshipers need to build them."
Cain was confused, "But this planet should be... Not worthy of a gods attention. I think..."
The old man chuckled, "If you think about it, then yes. We indeed are an insignificant pebble of dirt floating in an infinitely large pool. However, each planet a god possesses under their influence is a huge power boon. Most gods don't even have any mortal realm planets under their command. However, they might not come themselves. Even the lowest of their attendants will be enough to wipe us all out."
Cain sighed, "So these gods coming here is a bad idea. Got it."
The old man chortled, "If we can invade the temples and destroy these alters, we can wrestle power from that stupid figurehead sitting on the throne."