Competition. It’s all about competition between companies. In highly saturated markets ads serve to build brand recognition and are seen as long term investment to gain market share. Good example is TikTok. The social media market is very much saturated and competition very high, so TikTok’s strategy was to buy every ad they could to create brand recognition, which in turn helped gaining market share.

So I don’t think the ad business is a bubble at all, it’s just that the usefulness and function depends on the market a company competes within.

Also if you check your Google Photos privacy settings there is a toggle for “Approximate missing locations” which tells me that Google is most definitely also using private images as training data and not only street view.