I was a cryptologic linguist for the army and went through a 16 month course of 6-7 hours of classroom instruction per day M-F, native speaking instructors, tons of immersion, and a couple hours of homework each night and left still feeling like I hardly knew the language that I was supposed to be interpreting. Unless your major in college is a specific language, you probably won't achieve any sort of fluency in your target language from taking a course in it, either.

Language learning is hard and a multifaceted thing that no single application is going to adequately prepare you for. Speaking and listening are related skills yet speaking is much harder than listening, and both are much harder than reading (for the vast majority of languages). Rote memorization is necessary. Immersion is necessary. Comprehensible input is necessary.

Language learning apps like Duolingo sell the idea of learning a language which is an extremely attractive idea to a lot of people. They also make it a fun activity and incentivize engagement with humor and gamification. The problem is that you can't possibly become fluent in a language with an app like it and there isn't any single app that will.

I suppose in the end Duolingo's valuation is more a reflection of people's desire to learn a language rather than people actually learning a language, though.