Most people don't know that the Treasury Building is a big office building directly connected to the White House by basement walkways.

Because it's about Eisenhower Executive Office Building

Why is this omission of any note? I know the building and I can tell you that it is not particularly remarkable.

Because it's not the treasury building lol. There are multiple buildings near the White House that are under this umbrella, and I wouldn't think Treasury is one of them since they need their own office space.