This article by Robert Ralph explains why most publications want drafts. If an article is already published by the time it is added to a publication it might just end up in it's archive which may not help the publication or author. It won't be send out to the publication's followers or newsletter so it might just end up being published in the publication but ending up in the void.

https://medium.com/new-writers-welcome/where-is-my-article-that-was-published-f5888a25569c