Teaching a billion people ten words makes a lot more money than teaching 100k people 100k words. The stated goal is to teach _you_ a language but the actual goal is to be lightly used by as many people as possible.
Here's a crazy thought: People are different, and different people will have different best ways of learning a foreign language, depending on what they know beforehand, and how they like to learn.
Some people are having great success with duolingo, good for them. I'm not one of them, I think its structure sucks for me, I need way more grammar and structure rules, not an endless string of random sentences.
And the thing is, duolingo could add what I wanted to their lessons. They could absolutely add lists of pronouns or prepositions, or several sentences with small variations to their repertoire.
Get some basics and then practice by reading, watching stuff and most importantly: talking with other people. Duolingo is pretty good to get you to the start line.