RapidAPI does offer some of the tools that Kable offers. And if you're looking to monetize a single REST endpoint, RapidAPI is probably the right tool for the job.
Kable is infrastructure for API companies, brands, and larger projects. You wouldn't find Twilio's SMS API in RapidAPI's marketplace, nor would you find many of today's hottest new API-first companies. Kable's mission is to help those companies launch quickly and scale.