The part about added cards is just how many cards you have made in Anki, not how many cards you have learned. I haven’t learned 39,000 cards. I just have so many because of sentence banks from morphman if you know what that is. I probably only know like 6000-7000 words.
The main reason is that there are many words you will need to know to live in Japan (if you want to) that probably won’t appear much, if at all, in your immersion. For example, “health insurance card” and so on. Each specific domain has its own set of vocabulary, so unless you immerse in a wide variety of domains regularly, you won’t be reviewing vocabulary from other domains when you immerse. I also want to do the JLPT for employment purposes and you also need a lot of words for that which won’t often appear in anime or whatever you immerse in.
I also found that, especially in the beginning, reading and listening to isolated sentences taken from immersion many times rapidly in Anki every day improved my reading and listening speed a lot compared to only doing normal immersion. I had audio and reading cards for each new vocab word in i+1 sentences and native audio. It’s like spamming ideal comprehension zone content for listening and reading in a timed and tested environment. If I didn’t understand almost immediately I would fail the cards.