If nothing is preventing you from building a personal one then nothing is preventing you from building a centralized one. All you have to do is convince enough

If nothing is preventing you from building a personal one then nothing is preventing you from building a centralized one. All you have to do is convince enough people to use it. Google is essentially a centralized knowledge base. So centralized knowledge bases already exist and they're called search engines. There are also free blogging services that publish everything on the web to be indexed by Google. Blogger is even owned by Google.