Because of standardized naming convention, or lack of.
In the Britney example, there's no naming convention, only a vague approximation of the image contents.
This may work for you personal blog, but in large production environments, you don't want editors just "making up" image names without consistent naming convention.
Remember that the original digital camera image has no mention of Britney! At best, when the RAW image is converted, the general image name might be appended with "mtv_awards", but nothing further.
Same for video file naming. You can't fit all the subject matter of the video into its file name, you would instead maybe put the reverse date in the name and an abbreviated title. But to expect that to be used for SEO is not a good strategy.
In the Britney example, there's no naming convention, only a vague approximation of the image contents.
This may work for you personal blog, but in large production environments, you don't want editors just "making up" image names without consistent naming convention.
Remember that the original digital camera image has no mention of Britney! At best, when the RAW image is converted, the general image name might be appended with "mtv_awards", but nothing further.
Same for video file naming. You can't fit all the subject matter of the video into its file name, you would instead maybe put the reverse date in the name and an abbreviated title. But to expect that to be used for SEO is not a good strategy.