It has to do with analog TV & tape-based media distribution.
Captions follow a very specific standard & are required to be included on certain broadcast media.
To ensure captioning remains consistent across consumer devices, captions come in specific file formats, typically generated by a third party.
The files are delivered alongside media deliveries & contain timecode markers to sync the text up with the dialogue.
Before digital delivery of media for broadcast, each piece of media arrived on a tape. Each tape had the caption file embedded as lines of video, TVs could scan those lines as they came across in the broadcast signal, & display the captions.
On certain old televisions, you could see the captioning during the broadcast, it would appear as broken black/white dashes across the top of the image.
Any text included in a caption file is considered captioning. Any text outside of that file that appeared over video (including titles, alternate language translations, logos, etc), is considered a subtitle.