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.
Part of the problem might be that no one wants to pay anymore. I'm happy to pay for a course, but there is not a single in-person language course in my city left for the language I wanted to study.
I booked the the single remaining one last year and shortly before the start they announced they will not run it anymore and instead do online classes only. Apparently the rent is too high and it just isn't viable anymore.
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.