Android doesn't show advertising. Ads in YouTube get classified under YouTube's revenue and not Android.
reply1. Google uses Android user data to build ad profiles for Google search
reply2. Android comes bundled with chrome (and thus google search) as default. Google pays Apple $20B+ a year to keep Google search the default on Safari. That should tell you how valuable controlling Android is for Google's margins.
Like no-one at google seriously thinks they need to take a 15% play store cut to justify funding Android.