Because open Internet principles as understood at the time required that. Facebook used to be heavily criticized (see e.g https://www.google.com/amp/s/www.wired.com/2007/08/open-soci...) for locking data into their platform when it ought to be available on the open web. There was a pervasive sense that you should be able to authorize third parties to do anything you can do through the official webapp; the term “walled garden” was common for platforms that wouldn’t offer this level of control.
Indeed - we should remember that a good chunk of the complaints about Facebook are because they opened up an API to anyone who granted permission, as demanded by power users like us who wanted different services to interoperate seemlessly.
