I'm guessing it's mostly up to mostly tradition/momentum on how the formats where initially created and maintained.

Videos has (most of the time at least) at least two tracks at the same time that has to be syncronized, and most of the time it's one video track and one audio track. With that in mind, it makes sense to wrap those in a "container" and allow the video and audio to be different formats. You also can have multiple audios/video tracks in one file, but I digress.

With images, it didn't make sense at least in the beginning, to have one container because you just have one image (or many, in the case of .gif).