I don't understand at all why it would be left out.Simple.
Two different messages for two different circumstances.
My Direct Add message is phrased in the context of "we've added you to our mailing list".
My Welcome Message, by contrast, is phrased in the context of someone who found us and joined on their own, thanking them for choosing to join. That phrasing comes off very stilted in the Direct Add context.
For the custom text in direct add, I typically write somethingYou can still do that, customizing the introduction to the message, then following following that with a "we've added you" version of the Welcome message.
There's no need to pester the person with two separate emails that arrive simultaneously.