My guess is that it's relying on openai calls to do the hard work, and thus someone has to pay for them. I believe that GPT-3, which this relies on, is not open source and probably not easy to run on your device.
Microsoft announced on September 22, 2020, that it had licensed "exclusive" use of GPT-3; others can still use the public API to receive output, but only Microsoft has access to GPT-3's underlying model.https://en.wikipedia.org/wiki/GPT-3
Microsoft announced on September 22, 2020, that it had licensed "exclusive" use of GPT-3; others can still use the public API to receive output, but only Microsoft has access to GPT-3's underlying model. https://en.wikipedia.org/wiki/GPT-3