Because building a social network is hard. Not technologically, any half decent coding school grad can put out a basic Twitter clone in a week worth of coding... but you have to spend lots of money to get users (either via advertising or via developing features no one else has), and the larger you get the more you have to spend on moderation.
Apple markets itself as family friendly, which directly collides with the tendency of people to post copyright-protected content, abuse of all kinds, pornography and similarly disgusting content, and not to mention consumers of CSAM who will invade any new platform that crops up to avoid getting booted off in seconds.
Apple markets itself as family friendly, which directly collides with the tendency of people to post copyright-protected content, abuse of all kinds, pornography and similarly disgusting content, and not to mention consumers of CSAM who will invade any new platform that crops up to avoid getting booted off in seconds.