I turned down a job offer at a company that relied solely on twitter’s api in order to accomplish their goals. It was a sales lead generation tool that used a scripted approach to warming leads before handing them off to AE’s to bring home.

Within a year Twitter shut down their access and the company went under. That’s the day I learned not to trust another company to allow you to make money with their product permanently.

Developing a good and feature rich game engine which also runs performant is a huge effort. That alone can cost a good team 2 years at least. Even more if we consider todays graphic standards. That's nothing which smaller studios can easily deliver. So yeah, it's an obvious decision to buy a license for a proprietary engine, where a lot of work has already went into. That's just business and nothing crazy about it. Companies using services or products of other companies is pretty ordinary.

Until a few years ago there was barely any alternative afaik