Perhaps those who have already done it are trying to keep it a secret. Maybe they're afraid if too many people start doing it, the IRS will change the rules and disallow it.
If there was a way to do it a major investment firm would do it and make billions of dollars in a week.
Simply not true. Major investment firms do not do this for a couple of reasons. One is the risk of being trustee on an assets they can't "control", like a mutual fund or stock. Two is fees. You can charge an IRA fee for holding alternative assets but its a lot harder to convince an investor to pay an asset management fee when all Morgan Stanley, Goldman etc is doing is providing an IRA.