It's the same as Tinder. The business isn't about getting you dates, the business is selling you the fantasy of getting a date. Those two things are different, but for a new user, difficult to distinguish. Duolingo offers you the fantasy of speaking a new language.

Of course it is possible to learn a language using Duolingo, just like it is possible to get dates on Tinder, but it's just not a good method. If you're new to learning foreign languages, you'd be better off signing up for a course (but that costs time and money), and if this is your n-th foreign language, then you'd rather get a book and some boring flashcard app.