>this is why you sometimes have sign language interpreters at events, rather than just captions.

No, the reason is because a) it's in real time, and b) there's no screen to put the subtitles on. If it was possible to simply display subtitles on people's vision, that would be much more preferable, because writing is a form of communication more people are familiar with than sign language. For example, someone might not be deaf, but might still not be able to hear the audio, so a sign language interpreter would not help them at all, while closed captions would.