SGML might not have been designed for data serialization (it was designed for document authoring), but the XML standard committees and company representatives were heavily interested in generic data serialization.

And YAML was also designed for human readable editing and serialization of configuration among other data serialization needs. "Configuration files" is literally the first use case for it mentioned in the standard's homepage:

"Even though its potential is virtually boundless, YAML was specifically created to work well for common use cases such as: configuration files, log files, interprocess messaging, cross-langauge data sharing, object persistence and debugging of complex data structures".

But the point is moot. What each language was "designed for" is irrelevant to what it's predominantly used for throughtout the industry.

People found a use for them, regardless of whether their designers might or might not foreseen it (they had), and for this use which is what interests people, there are certain issues.

If the answer was as easy as "just use a language better designed for that use case and it will solve your issues", they would have done it already.

Some have their hands tied because vendors/projects/etc they use, enforce TOML or YAML or XML and have to use them too to work with them. Others find alternatives worses for their use case, but still don't consider the one they use (TOML/YAML/XML/etc) as optimal.