Because every god is a god of power, especially power over death. And if God is for us, who can be against us? Even Nazis had "Gott mit us" engraved on their belts. Nothing is more effective in uniting a country in war than believing they are fighting in the name of God and that the enemy is a bunch of godless infidels that they are obligated to destroy.

Part of the business model is the promise of salvation, but to make that a palatable incentive fear mongering is essential. Just watch old movie where mobsters intimidate shopkeepers and than sell their protective services. Guess where the Mafia copied the template?
The three major monotheistic religions with all their tentacles promote Armageddon as the final solution. Then they give you the contract for a ticket. First prize: One way to Paradise everything included a sort of heavenly Club Med. Some religions at least tell you that there is a finite number of prizes avaiblable.
Religions are the main promoters of war.

True religious believers feel they must convert everyone to their way of thinking. ("If they are not with me they are against me" ). Religious believers feel that any other religious philosophy that doesn't agree with their own is evil and must be destroyed. Their God is the only true God after all. Hatred, malice, and hostility are born from their self-righteous belief system. Murder, genocide, and war follow.