AU Details

Because a DETAILS coin has no grade.
It just has an "appearance".

I can think of 2 possible reasons why they don't give a number with their details grade. Grading problem coins can be very tricky. Let's say we have 3 coins all with the same amount of details: one is a well struck EF, another is less well struck AU with mint luster and a little bit of wear, and the third is a weakly struck but fully lustrous MS. If the surfaces of those 3 coins were abraded like through cleaning, whizzing, smoothing etc, they can look very similar to one another making it challenging to distinguish which was once the EF, AU, and MS. Those grades cover 30 pts so imagine picking between grades that differ from between 3-5 pts.

The second reason I think has to do with financial incentives. The added difficulty to grading problem coins translates to additional time to come up with a grade and describe the problem. In the end it will mean fewer coins will be graded. PCGS had at one time slabbed problem coins without mentioning a details grade but I guess they got enough pressure from the marketplace that they began a system like what NGC, ANACS, and ICG were already using. So for these reasons, is why I think they chose this way to grade problem coins.