This is largely true because this is where the largest volume of traffic comes from; however, it's not exclusive to these two by any means. There are some pretty big Supply Side Platforms and aggregators out there for advertisers to use. This comes into play a lot more often on podcasts and streaming audio, in particular, consider the fleet of Amazon Alexa devices out there.

Many of the advertisers that sell on these platforms are quite familiar with buying ads directly from "old school" media companies. So they have the competence and familiarity to be put off by the metrics but are apparently not in a position to force Google and Facebook to match standards used in other contexts.