ffmpeg – Why the video contains no SAR and DAR value?

SAR(sample_aspect_ratio) and DAR(display_aspect_ratio) are stored in video’s metadata information.

ffprobe -v error  -i  sample1.mp4 -show_streams  |grep aspect_ratio
sample_aspect_ratio=1:1
display_aspect_ratio=203:360

Why some video such as sample2.mp4 contains no SAR and DAR value?

ffprobe -v error  -i  sample2.mp4 -show_streams  |grep aspect_ratio
sample_aspect_ratio=N/A
display_aspect_ratio=N/A

Video info in sample2.mp4.

ffprobe  -hide_banner  -i  sample2.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'sample2.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.20.100
  Duration: 00:16:33.33, start: 0.000000, bitrate: 625 kb/s
  Stream #0:0[0x1](und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709, progressive), 406x720, 573 kb/s, 24 fps, 24 tbr, 90k tbn (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]
  Stream #0:1[0x2](und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 47 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
      vendor_id       : [0][0][0][0]