why some mp4 video files only audio

6:37

2:44

00:55

5:59

3:01

3:59

3:54

6:00

2:09

6:21

1:37

5:08

3:33

8:20

1:33

3:13

4:12

4:37

4:29

1:29

2:49

1:20

2:46

2:26

1:47

1:10

3:26

12:08

00:54

6:42

5:04

2:21

1:03

2:34

2:51

5:38

1:06

9:59

3:02

2:35

3:55

7:11

2:10

00:58

3:00

1:52

1:22

3:34

9:15

1:08

2:44

4:12

5:31

5:27

4:27

6:52

4:47

1:40

1:50

2:58