It's really funny how the "singular, omnipotent god" religions fucked up the logic behind prayers. When you had many gods and spirits and whatnot, a prayer not being heard could be taken as it not being stronger than another spirit/god, or the god of your choice ignoring you, or something else.

With omnimono, god has a plan, so prayers are either trying to convince him to change the plans (which, if assumed true, opens a very interesting can of theological worms), or will fall on deaf ears, as the plan won't change.