It makes sense - Google can't run YouTube without ads. Ad buyers, which have ads in video form, don't want to run ads when the user isn't looking at the content nor able to easily click on their link to convert them to a paying customer (plus google never gets paid as the user probably won't switch to the app just to click the ad). They either do this or ask advertisers to make ads specifically for audio-only streams (which still makes it hard to drive conversions), but then they'd have to charge advertisers for impressions which Google has very rarely done.