Now true they could do that and not make it available to advertisers but the very idea is pretty fraught.

More than fraught... platforms are often contractually prohibited by the streaming apps from collecting and analyzing actual viewer data. Usually the meta-search and meta-merchandising (ie "recommendations") are built from some agreed upon set of rules between the two companies. They often include some watch history, but it may be the streaming app feeding that into the platform rather than the platform doing the analysis. Contract terms are different for every provider though, so it's a big ol opaque mess. Rarely just a recommendation algo at work.