The actual reason is that the vast majority of Duolingo users are only using the app because they like to play Duolingo. I don't say this to insult Duolingo users, but they're not actually serious language learners. They just really like playing Duolingo and if Duolingo stopped existing, they would simply stop "learning" languages all together.

Sorry, maybe I'm not using it right in learning a language, but isn't Anki just flashcards?

because setting up and using anki is a pain? using and configuring anki is not intuitive. if you're just starting, you'll probably need to find a good premade vocab deck. once that's done, you'll need to make cards. making cards manually is a pita, i would probably not make any if i didn't have a good mining setup (yomitan/ttsu reader/asbplayer). i've spent countless hours getting my mining setup just right (setting up anki and yomitan, tweaking card fields/templates). yomitan probably works well because japanese conjugation is so regular. i imagine for languages with less regular conjugation, it wouldn't be so nice.