Because it's possible to select notification schemes that are inappropriate. Some sites I've been to, it would work fine. At most, it's not - most project admins quite happily select schemes that they simply don't understand, and end up spamming all sorts of people (It's not just notification schemes, most schemes are too intricate to be trusted to project owners to select the right one, but notification schemes are definitely the ones that cause the most complaints from the users because of the spam an incorrect one can generate).
There's no way to change it without editing the code for project administration.
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