This is a misunderstanding of what these cohorts are. If you read the [FLEDGE readme](https://github.com/WICG/turtledove/blob/master/FLEDGE.md) you can see that there are multiple types of owners of cohorts that fall into 3 categories:
1. Advertisers - who would add you to a cohort they own, so say "Nike - womens-running-shoes"
2. Publishers - who'd want these cohorts to better allow for advertisers to advertiser on their site, so "NYTimes - business-section-reader"
3. Third party ad-tech companies looking to create audiences for advertisers who work with them, so "Example Agency - mens-formal-wear". They'd partner with publishers so that when you go to somewhere like GQ or Man of Many and read an article on tuxedos then you'd get added to the cohort.
So you are right, you'd never see a prompt to add in your interests, but this isn't because Google doesn't want to associate with specific brands, it's that Google isn't necessarily an owner of a cohort (though they totally could create their own under any 3 of those categories).
1. Advertisers - who would add you to a cohort they own, so say "Nike - womens-running-shoes"
2. Publishers - who'd want these cohorts to better allow for advertisers to advertiser on their site, so "NYTimes - business-section-reader"
3. Third party ad-tech companies looking to create audiences for advertisers who work with them, so "Example Agency - mens-formal-wear". They'd partner with publishers so that when you go to somewhere like GQ or Man of Many and read an article on tuxedos then you'd get added to the cohort.
So you are right, you'd never see a prompt to add in your interests, but this isn't because Google doesn't want to associate with specific brands, it's that Google isn't necessarily an owner of a cohort (though they totally could create their own under any 3 of those categories).