I think this is not really true. They did all sorts of things… like, these fairly poor (by modern standards) people sacrificed valuable resources to their gods. There is no particular reason to think they believed in their gods any less than current religious people.
I know people with tens of thousands of dollars of Marvel paraphernalia. They spend thousands a year on tickets, events, comics. These people are not well off, it's money they otherwise would do well to have in retirement savings. Humans are not always rational.
They had big rituals that cost them a lot. I could see these as being performative. But then, for something to be performative, the people it is being performed to need to believe in it, right? Like modern generals don’t perform a sacrifice to Iron Man because modern soldiers don’t believe it is necessary.
They also had boring little rituals that weren’t really very effective performative signals.
What reason is there to think they didn’t believe in their gods? It is hard to query what’s going on in the heads even of living people, let alone long-dead ones. But I think the null hypothesis should be that people in the past at least believe their religion as much as modern ones do.
The comparison I would make is Santa Claus. He is not all powerful, but he has a lot of supernatural powers. He makes demands of your behavior (but you don't have to align your whole life around him) that comes with a tangible reward (presents). There are big, expensive and complicated rituals relating to him.
