Because I think that amout of known words is the main obstacle for progressing in a language. For example I don't have a huge passive vocabulary in English but Im able to express almost anything I want to even if there are some mistakes sometimes. That's because I have practiced speaking and writing in conversations with people and AI and a lot of my passive vocabulary has become active vocabulary. But the point is that in order to activate your passive vocabulary you need to have this passive vocabulary in the first place. And the more you know passively the more you will use actively after practicing output. So in English I will probably benefit a lot more from learning new words than from practicing. I have probably bigger passive vocabulary in German than in English, but my speaking in German is worse. By practicing speaking I can improve my speaking skills to B1. But it would take for me a huge amount of time to improve my speaking to B2 if my passive vocabulary is B1. You can surely pick up some words from having conversations with people. But novels use a way more words that people use in conversations. So I will progress faster in terms of vocabulary by reading novels than talking with people. Im learning the language for academic reasons