Because non-open ones are not available, even for a price. Unless you buy something bigger than the "standard" itself of course, like a company that is responsible for it or having access to it.

There is also the process of standardization itself, with committees, working groups, public proposals, ..etc involved.

Anyway, we can't backtrack on calling ISO standards and their likes "open" on the global level, hence my suggestion to use more precise language (“publicly available and sharable”) when talking about truly open standards.