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…
the problem isn't that it's hard to do. the problem is that their business model relies on their app not achieving it's portrayed purpose - learning. success means losing your customers. that's why I don't use it anymore and I'm baffled by how good they are at being bad. or rather - staying on the exact thin border between "not good enough" and "not bad enough".
Duolingo is extremely popular because people get use out of it and like it. You've taken a tiny perverse incentive and jumped to the conclusion that it trumps all the other more important incentives.