This is likely (directly or indirectly) the fault of a single paragraph in IETF RFC-7159 / RFC-8259 (shown in the attached screen-shot).
(And note that, there is a difference between JSON and IETF JSON. JSON did not have this. IETF JSON does.)
That paragraph (in the IETF RFC) was NOT a requirement. But, others made it a requirement — including JSON-LD.