Producing descriptive audio isn't always easy. Some content may contain a lot of speech without any natural space for a narrator to describe the visuals. If the intention is to offer maximum accessibility to the blind and visually impaired, the best approach is to 'pause' the video. The narrator now has an opportunity to describe what's happening before the film resumes. Approaching the AD this way can make the video much longer. Maximum AAA accessibility, in this case, is also more costly since there will be:
more for the video describer to write
more for the narrator to record
additional video editing work required
Of course, the visually impaired audience will appreciate the extra effort, and they will especially love our expert narrator voices.