This topic is discussed at Commons_talk:Structured_data#Proposal%3A_Move_captions_from_file_information_to_structured_data_tab

While the captions are indeed structured data, they also represent descriptive information in a format that is more human readable than the rest of the structured data fields, which is why they are in the main "file information" tab. For users who are not fluent in English, the caption may also be the most easily accessible descriptive information available in their language. I do think that clearer modeling of how to use the "caption" field would benefit everybody. If you could say more about how having captions on the main page gets in the way of your workflows or of easily viewing file information, or if they represent some other challenge I don't understand, I'd love to hear about it. We're certainly open to making a change if users would prefer to move the captions to the structured data tab. However, the team has other priorities right now, so I'm not sure when we would be able to focus on it.