Because Duolingo isn't enough. I had two years of Rosetta Stone before I went to China. It was just enough for me to survive, but it was not enough per se. Duolingo isn't any better. It's been useful to keep my skills, get some vocabulary, but it isn't enough!
You need to actually practice.

Really? I did a 365 day streak in Japanese (and most of that time was spent learning Hiragana and Katakana) but I still had some basic phrases down at the end.

Kanji broke me in the end though, Duolingo really doesn't do it well.