This isn't some new "problematic trend". A website serving all kinds of requests from a single domain name has been how things have worked forever.
Pages making cross-origin requests is in fact a new phenomenon, and has been widely adapted by the ad industry simply because the ad server cannot trust the content host to report its own ad view/click-through numbers. If the server doing the data collecting or ad serving is the same as the one providing the content, there's zero reason for them to be on different domains. And for all these cases any kind of network-level blocking is always going to be ineffective.
Pages making cross-origin requests is in fact a new phenomenon, and has been widely adapted by the ad industry simply because the ad server cannot trust the content host to report its own ad view/click-through numbers. If the server doing the data collecting or ad serving is the same as the one providing the content, there's zero reason for them to be on different domains. And for all these cases any kind of network-level blocking is always going to be ineffective.