It's like in martial arts you learn kata first, though if you get to apply it later you don't do the same kata, you do something else - but to do it, you have to learn to do kata first.
That's kind of their approach. I agree that in case of Japanese they could do better. E.g. if they explained what is explained here: https://8020japanese.com/japanese-sentence-structure/ - and they have space for such texts, but they are not using it for Japanese - it would save me some frustration and trying to figure out what these weird things are doing inside the phrases. Once you get the principle, it kinda clicks in place. So with the help of some other sources, so far it's not too bad for me with Duo and Japanese I feel. Though of course I don't think it's possible to use it alone without some auxiliary materials.
Well, if you're coming from a European language background, Japanese is one of the hardest languages to benchmark on. I mean, there's three writing systems (hiragana, katakana, kanji) for starters! And then unlike European languages, it's not alphabetical and kanji is not even really syllabic - which means, the background you need to get to be able to read texts is huge.
E.g., for Spanish, I can read the newspaper (not understand yet, but just reproduce, look for words I recognize, etc.) without even touching a language course. And if I get the basics, I can see grammatical structure, and given common Romance background, get about 20% of vocabulary for free just from knowing another European language (well, not true for all European ones, but for many it is).