It was originally designed as a game asset storage format[1], wasn’t it? Which is why the reference implementation assumes trusted input by default[2], for example, unlike Protobufs, Cap’n Proto, or SBE.

[1] https://opensource.googleblog.com/2014/06/flatbuffers-memory...

[2] https://google.github.io/flatbuffers/flatbuffers_guide_use_c...