APIs cost more than people think. On the small scale it doesn't seem like much, but then you have to account for the abuse cases. After that, you have to deal with the regular scaling, and eventually legitimate high-volume users. After a while, maintaining the API itself becomes a project of it's own. Keeping it cheap as your product gets bigger is another task, and maintaining parity/versioned endpoints adds more pain on top.
For a lot of products, the cost-benefit analysis doesn't align with exposing unlimited free APIs. We see this pretty often with data that scales beyond simple static metrics like weather updates or RSS feeds.