Same reason the iPhone doesn't ask you which App Store you want to use. If you disagreed with the default choice, you'd be using another phone.
That's how technology is nowadays, unfortunately. Pick the tribe you want to support, and watch them blatantly disregard every standard and convention of modern society. Your best bet is to personally divest from these ecosystems and advocate for their regulation and dissolution, if you think it's bad.
I understand where you're coming from, but what you're describing is why they don't want or have to, not why they can't, which is specifically what I'm focused on as I think we otherwise just take these being the same thing as a baseline assumption, which is obviously not the case.
We need to be clear that this isn't a technical limitation on the part of these companies, and them choosing to pursue this modus operandi is purely for their own convenience at the cost of the user.
That's how technology is nowadays, unfortunately. Pick the tribe you want to support, and watch them blatantly disregard every standard and convention of modern society. Your best bet is to personally divest from these ecosystems and advocate for their regulation and dissolution, if you think it's bad.