It's due to fragmentation in implementations. The YAML spec is fairly large, and library authors understandably aren't excited at the prospect of dealing with all the minor details of less commonly used bits, like anchors.

You end up with YAML libraries being widely available and they all implement _most_ of the spec, but the portion not implemented varies. Plus there are minor variations in how people have read the spec, and even options _within_ the specification on how to handle parsing:

https://yaml.org/spec/1.2.2/#chapter-10-recommended-schemas

This isn't really a problem for projects like Kubernetes where most of the ecosystem uses the same library. On the other hand, OpenTelemetry is rolling out "declarative configuration" across all the supported languages. I work on OTel, and have avoided contributing in that area because I lack the patience to deal with the inconsistences - I don't hate YAML, but it can be frustrating.

AFAIK, the only 100% spec compliant implementation is libfyaml.