Private standards are also how you end up with a variety of implementations that are all non-conforming in their own subtle ways. Some because the authors didn't have access and were attempting to do the best they could with documentation from other implementations, third party articles, and reverse engineering. Some because the authors have the standards, but the consumers of their products don't and won't hold the authors accountable since they don't know how it is really supposed to work anyway. Others because they think there is competitive advantage in deviating from the standard.

It's hard enough getting consistent behavior out multiple implementations of the same public standards.