Because the lowest common denominator is much MUCH lower than you think it is.

This means it's easy to indoctrinate and easy to maintain that for a massive number of people.

Scientific illiteracy is extremely high, and actual "6th grade reading comprehension" is the highest level of literacy for > 50% of a country like the U.S. and ~20% are low literacy or actually illiterate.

This means that half of everyone in the U.S. can read and understand what they read at or below a 6th grade level. This isn't "reading big words", it's "tell us about what you read", "what is the relationship between x & y" type questions.

This comment for example, up to this point only, would be difficult to understand & comprehend for > 50% of people in the U.S. (it demands an 11th grade reading comprehension). And may be misread, misunderstood, or not understood at all.

People are driven to religions to cults and alt conspiracy theories when they don't understand how the world works around them. They latch onto extremely simple often misleading or incorrect ideas of how the world works because they can understand it and it "makes sense" within their sphere of ignorance (we all have one, this isn't meant to be a disparaging term).

This means that the problem is that humans are just not smart enough to escape religion yet. It's the simplest answer, and it appears to be correct.