No because YAML is full of ambiguity.
JSON5 is not.
There's something to be said for being flexible in your inputs when they are non-ambiguous. Particularly when dealing with files written by hand.
There's no virtue in imposing overly strict syntax when it serves no human purpose. That's trying to alter people to fit machines, rather than altering machines to fit people.
Yeah, and we could easily say that any property value without a comma, closing bracket, or closing brace; that doesn't follow the format for numeric literals; and isn't the literal value 'true' or 'false' doesn't need to be quoted when part of a value and will implicitly be a string. This adds no ambiguity and some people are used to writing their string literals without any extra quotes, why add the extra cognitive load of having to remember to use quotes? Wouldn't it be nice to write `user: { type: admin, name: Bob }`?
This seems like exactly how we ended up with YAML-and-the-kitchen-sink.
For a spec like this, things should need a better justification than that. Everything starts at -100 points. How does this feature justify the additional complexity in the spec, to the users, and to the people trying to implement this? How does it justify increasing the opportunity for creating subtly incompatible parsers?
Or, if a goal of JSON is not to be simple and interoperable, then I fail to see how it's not just YAML but 5 or 10 years behind the curve, so we may as well skip a bunch of hassle and move to YAML and start fixing the bugs we've got there rather than creating new ones here.