Because we don't understand what "senior administration official" means. The significance of the document depends on the authority of the author. The NYT has implied that the author is substantially authoritative. It would be nice to evaluate that for ourselves.
The reasons are well understood. What is also understood is that there is a trade-off here. Keeping the source of the piece anonymous provides both advantages and disadvantages. One disadvantage is that it weakens the authority of the piece. If you want me to seriously consider that someone in the Cabinet is actively obstructing their president, then you have to show me the actual Cabinet member that's doing so. An anonymous editorial by a "senior official" isn't enough.
Again, this isn't really a big deal. Unless it's a big deal to you that I believe the author is a Cabinet official. If it's a big deal to you, then by all means, let's discuss it. If it's not a big deal to you, then you probably don't need to waste any more of your time on this sidebar.
Except that there is corroboration of the central idea
And of course there is the draft letter that Garfy Cohen took from his desk - which provides corroboration
https://edition.cnn.com/2018/09/06/politics/trump-woodward-book-trade/index.html
Unless you think that taking letters from Trump's desk to prevent him signing them is working with him.
I'm pretty sure that wouldn't have worked with the previous four presidents

