I went to law school and got a job as a tax attorney. I don't consider my work or world particularly secretive. The barriers to entry are not peculiarly high and lawyers as a profession are a little too happy to talk about themselves.
There's a great comment on the Atlantic that states this better than I can
"If by secret, you mean extraordinarily complex, then yes. But... that's not the definition of secret.
It's not secret. There are many classes on this (albeit from professional-level degrees) and it's all based on public statutes and court cases. All of which are freely available to any person.
A large part of it being "secret" has to do with people's willingness to listen and understand. I'm a tax attorney, I understand these deals, and I can explain them rather well so even someone unfamiliar with tax law will understand it. However, it still does take a bit of active listening on behalf of an unfamiliar (as in, unfamiliar with tax law) listener to actually understand what I'm talking about, regardless of how much I simplify it."
The most disappointing thing about international tax is how little even professional journalists understand it, or care to. It is simply easier to assert that international tax is a scam and since the general public can barely count to 10 using both hands, there's no way to disprove them.