I agree. I don't understand the driving force here.
I have all of the raw image files that I've uploaded to Instagram. I can screenshot or download the versions that I created in their editor. Likewise for any text I've published anywhere. I prefer this arrangement, where I have the raw data in my personal filesystem and I (to an extent) choose which projections of it are published where on the internet. An IG follow or HN upvote has zero value to me outside of that platform. I don't feel like I want this stuff aggregated in weird ways that I don't know about.
For me, part of it is that we have no power collectively against products turning their back on users because coordination to "export data all at once and then import it into specific other place" is near-impossible. So this creates a perverse cycle where once you capture enough of the market, competition has very little chance unless they change the category entirely.
What AT enables is forking products with their data and users. So, if some product is going down a bad road, a motivated team can fork it with existing content, and you can just start using the new thing while staying interoperable with the old thing. I think this makes the landscape a lot more competitive. I wrote about this in detail in https://overreacted.io/open-social/#closed-social which is another longread but specifically gets into this problem.
I hear you re: not wanting "weird aggregation", that just be a matter of taste. I kind of feel like if I'm posting something on the internet, I might as well have it on the open web as aggregatable by other apps.