From Meta's perspective, in all the ways that matter, the advertiser is the end user. Non-advertisers' impressions and data are simply inventory that can be sold to end users. And it would be bad for the "end user" if that inventory was stored by default in a form that could not be easily indexed for cost-efficient packaging and delivery.

(Lest you think I'm exaggerating, inventory is literally an industry term: https://smallbusiness.chron.com/advertising-inventory-mean-3...)