Everyone agrees that the absolute best way to learn a foreign language is immersion. And nothing beats constant exposure to conversing in that language. A recent discussion here on Duolingo even validates that.

So...how/why hasn't LLM completely upended this market?

If I had a Spanish LLM that I could converse with (either spoken, or text) for an hour a day I'd likely get WAY more mileage out of it then a gamified UI full of badges and leaderboards and artifice.