Comment by PokestarFan

This is because blurays ship their subtitles as a bunch of text images. So pirates have 3 options:

1. Just copy them over from the Bluray. This lacks support in most client players, so you'll either need to download a player that does, or use something like Plex/Jellyfin, which will run FFMpeg to transcode and burn the picture subtitles in before sending it to the client.

2. Run OCR on the Bluray subtitles. Not perfect.

3. Steal subtitles from a streaming service release (or multiple) if it exists.