Presumably the same justification/reasoning could be used to serve a home/business search warrant.
In theory you can get this reversed, but it is common for courts to uphold the government's interest in keeping investigations secret well beyond normal disclosure dates, in a few cases even up to and during the trial against the person who was served the warrant.
As to your second sentence, I think it's very reductionist. I see a FISC warrant as functioning like a subpoena. Say the SEC subpoena's your company's financial records from your accountant, and finds in there evidence that is sufficient to get a proper warrant to get documents from your company's premises. Does that mean there is no protection or purpose to the SEC not just being able to give itself Article III warrants?