When using stdin as input with FFMPEG, why does the resulting video not have a duration?

edit

When using stdin as input with FFMPEG, the resulting video may not have a duration because FFMPEG cannot determine the exact length of the input stream until it reaches the end of the stream. Without knowing the exact duration of the input stream, FFMPEG cannot set the duration of the output file. Therefore, the resulting video may have a duration of 0 or may not have a duration at all.