Here is what TV does have: a compelling narrative. The narrative is about GRPs and the impact of good creative (that is, the fact that advertisers can tell impactful stories with commercials.) We do have online GRPs, and they are fundamentally the same metric as on TV: impressions divided by population, X 100. All these things are easy to count (easier online than on TV, in fact). But the digital GRP narrative has simply not proven to be compelling (save for on the part of a few very large publishers.)
Good point but this is only part of the problem. The other is that consuming online media is a fundamentally different experience so I’m not sure online will ever get to where TV is, at least in the current paradigm.