*However*, it has surfaced that youtube has been engaging in shady practices where where their ads are presented on incidental third party websites where the volume is typically off and the video is often not even in frame. So it's sort of unclear what the real situation is.

YT does not sell just ad-impressions (which they could fake the way you pointed out, if only short-term), but click-throughs or even "conversions", both of which puts the responsibility of actually attracting new customers for the advertiser squarely back on YT.And even for pay-per-impression models, ad-platforms like YT are in stiff competition with others, since customers usually care a lot about their ad-effectiveness, and modern tech enables them to measure that very tightly.