Just simple explanation why it is fraud.

Suppose that you have village and they produce 10 baskets with various food per day. For 10 people, and each of them pay totally 25 coins per basked to all other 9 people who produce food for his basket.

Now our modern swindler come. And he declares that you could become rich fast. But you need to buy his magical algorithm on how to carve prime numbers from stone. It is very important numbers, he tells you. Each number cost no less than one complete food basket. But it is time consuming to calculate and carve it all, but it is worth it, as you can calculate no less than two per day instead of stupid food production.

Fraud here is that swindler attached value to thing lacking it as he did not produce food or anything else useful and he also made big damage by rerouting people time and energy into pointless time waste. His interest here is that he already have lot of worthless stuff, and he goes from village to village to get food and exchange it for nothing.

On big scale to it also adds Ponzi scheme, of course.