It comes down to cost and what LEGO thinks would be profitable. I think it would be safe to assume the designers were given a price and parts count to work with and had to create the HQ within those guidelines. I'm sure the back was not intentionally designed to look awful, but perhaps making it match the rest of the building without sacrificing the interior exceeded the budget? The reality is, most collectors are going to either display it opened or the front facing towards them with the car driving out.
I get that people want perfection, especially at this price point, but I think you need to look at other sets released by LEGO and have realistic expectations. Very few are universally loved and considered perfect out of the box.
To whose standards? Is the finish of the back of the HQ really necessary to the point where the set is neither enjoyable or functional in anyway? Do you get upset looking at the bottom of the Ecto-1 because it doesn't look like the bottom of a real car?