There are a couple of answers to this:

1) Most pro-MLMers don’t know any real secrets to becoming successful in MLM. They cannot teach what they do not know.

2) If everyone knew the secrets to becoming successful in MLM, what would be the value of those secrets? There’s a huge captive market for books and audio/video training claiming to offer success tips. The secret is valuable, in part, BECAUSE it’s secret.

If everyone knew the REAL secret of MLM, it would all unravel.

Which leads to your next comment…