YAML is just a markup language. Literally 
YAML = yet another markup language
It doesn’t provide any functionality, it is just the way to organize your metadata.
consider
---
family:
mother:
name: Mary
age: 50
father:
name: John
age: 55
---
then we can use it somehow like frontMatter.family.mother.name
Can it be rewritten without multi-level YAML? Sure, it can
---
family_mother_name: Mary
family_mother_age: 50
family_father_name: John
family_father_age: 55
---
and be used like frontMatter.family_mother_name
But for me it looks less readable, tons of repetition
Nestedness is the key concept of YAML, not supporting it, in my opinion, is a drawback
And I am not even talking about other more advanced YAML techniques such as references…