I am sorry, but you are wrong. MacPorts will not go through configure/build/install for all ports, only those for which no binary archive can be made available. As others have already explained, this happens when license restrictions do not allow redistribution of a binary.

With your example of ffmpeg, you can check yourself that ffmpeg-4.3.2_0+gpl2.darwin_18.x86_64.tbz2 exists as a binary archive and will be used on a standard install on macOS 10.14. MacPorts will definitely not build ffmpeg from source by default.

I recommend you test again with something simpler than ffmpeg, for example bzip2 or less.