This type of feature looks simple, till you try to implement it, then all sorts of issues arise.
You can't rely on "the quote button has been clicked", the poster may change their mind and edit the quote out before posting. Or type the quote manually.
That leaves you parsing every post for quote tags, and on finding them, parsing the quote tags for member IDs, possibly doable, but not that simple.
Then you have to decide how to handle nested quotes, notify the first? All persons quoted? How many levels?
Then after you've crafted these custom changes to the board's php, you have to check that every security update the upstream maintainers issue doesn't break your custom feature. Such updates happen quite often.
I suspect the answer boils down to:
Hypothetical Board Maintainer wrote:
It's too much hassle to do well for a non vital function.
Oh, ok. I think I understand, thank you