It's not just how easy it is to embed, it's also really small in both code + runtime size. I've shipped it on systems with sub 8mb of total system memory(we used a preallocated 400kb block), until quickjs came along there really wasn't anything comparable. It was also much faster than anything else at the time and regularly beat v8 in the benchmarks I ran.

Unity+Unreal are the public engines out there but there's plenty of in-house engines and tool chains you don't really hear about. I wouldn't be surprised if it's still deployed in quite a few contexts.