same reason wikis arent, it takes too much effort to maintain and keep it semantic instead of copypasted pile of text
you'd need CMS, CRM, knowledge base, documentation, source control, chat/forum, inventory, point of sale, customer support channels, issues ticketing system, and every other thing interconnected to with each other
do you know of anyone offering this as a complex turn-key integrated solution at a reasonable price?
if all you wanted to do is look around, it is understandable why giving up was this easy.
honestly, if github and HN suffice for all your social needs, you probably dont need anything else.
microblogging is not for everyone. but for others, since twitter is just no longer viable, people have been migrating to something that can't be just bought out by yet another rich asshole. plus privacy, extended control over your own data, and all that other nerd shit no one cares about.
there are multiple server-side platforms too, incl. some forks, so if mastodon.social is not for you – look around for better lighter alternatives (as well for self-hosting)