Show HN: Index funds are better than Managed funds?

Kind of a mix between show / ask:

If actively managed funds give higher returns than index funds, why don't fund groups offer a product that guarantees index return plus a spread (say 0.5%). Charge a fee to get in. Invest the money in a way that beats the market. Their profit would be fees + spread over market.

This seems like easy money if you know you can consistently beat the market. Does this not exist because fund groups can't beat the market?