It's beyond me how they still refuse to offer an official API.

Gemini subscription costs as much, allows me to do much more, and I can call it from vim or emacs without some arcane acrobatics

They used to have an API, which they removed about two years ago or so. Until then, there were a bunch of 3rd-party Grammarly plugins, including a pretty decent one for VSCode.

Mainly because (cloud-hosted, bigger & smarter) LLMs probably offer no added privacy benefit. Self-hosted models are dumber, neutered and unpredictable.