Blu-ray allows for text subtitles and there were a few rare discs that had them. The problem with letting the player render text is that it can lead to wild inconsistencies even if they all claim to follow the "standard". Just imagine every player doing fonts, colors, language, sizes, placement, word wrapping differently. Even our PCs often screw up text handling especially when multiple languages are involved.
When you store subtitles as graphics you can guarantee they will look exactly the same on every player.
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