Consider this, WhatsApp stories are not much different than personal blog but putting up stories takes few clicks while self hosting is whole new endeavor. Ideally everyone should own a blog/self host. This would solve the issues with centralization.
The problem is hosting a blog and discovering it is still not as easy as creating WhatsApp/Insta stories. Nor the users are ready to pay the price for running that blog. Centralized services solve all these problems. If some platform ever solves issues with self hosting and makes it easy to self host for minimal cost, I think we will have changed the face of Internet forever.
tl;dr We haven't achieved the required level of software/hardware abstraction for everyone to self host