Unfortunately, some apps don't support text selection and on some websites the text selection is unpredictable.
I'd actually compare screen OCR to screenshots. Instead of every app and every website implementing their own screenshot functionality, the system provides one for you.
Same goes for text selection. Instead of every context having to agree on tagging the text and directions, your phone has a quick way of letting you scan the screen for text.
To be fair, I still use the "hold the text to select it" approach when I want to continue with the "select all" action and have some confidence that is going to do what I want.