Because Apple has already implemented do-not-track, and from what I read, CPM is cratering. That leaves the ad industry with three options:
1. Adapt to the new, anonymous normal. Targeted ads will no longer be a thing, so ads will have to be based on the content viewed, not the viewer.
2. Invent a new way to track users. I suspect many are hard at work on this already, and is definitely Google's plan. In Google's case, the tracking will look more like aggregate data than a fingerprint, but the end result is still targeted advertising.
3. Go out of business.