The whole FlatBuffers toolchain is wildly immature compared to Protobuf. Last I checked, flatc doesn’t even have a plugin system - all code generators had to be upstreamed into the compiler itself.
The whole situation is extra confusing because some of the core codegen plugins are built into protoc. That's a distribution and version-matching strategy from the Google team rather than a requirement of the plugin system. I'd very much like Google to ship the built-in plugins as standalone binaries, even if they're also bundled with protoc.