I have thought about it and I think it makes it easier for frontends as they can simply statically link with it, and it also makes runtime decisions on which backend to use simpler. If the unixlib fails to load, or was not built with FFmpeg support then it will be reported to the callers and they can either return the error or try to use something else.
Also, I have tried implementing alternative decoding path for cases where the system FFmpeg is crippled, for instance targetting macOS videotoolbox directly, and I don't think it's necessarily limited to FFmpeg, though it still uses libavutil because it's simpler to use the AVFrame/AVPacket structures internally.