Short answer: it's not a logistic possibility
Long answer:
As many of you know, I am the sole developer for renshuu, and do everything else in the company aside from the artwork (which my wife Saki does). renshuu is over 20 years old, and for most of that time, was only a website (renshuu.org, which you can still use now alongside the app!)
As a company (and as a family), we have a few simple goals:
1. Help as many people as possible learn Japanese, for as little money (free for most people) as possible
2. Make enough money to support our family, but no desire for massive growth
3. Keep renshuu small enough that I do not need to hire staff and become a larger company
With this in mind, the the apps were developed as extensions of renshuu.org, and not standalone apps. renshuu's codebase is probably closing in on 1,000,000 lines of bespoken code. Because of that, I maintain a single look and layout for renshuu across iOS, Android, and renshuu.org. This gets me a lot of criticism about the look and feel of the apps, but it allows me to keep renshuu ad-free, and free for everyone that wants to learn for free, with renshuu pro being 100% optional. We have no desire to change this.
Even with cross-platform developmental systems such as Flutter (which I use) or React Native, making even a part (lookup tools, quizzing, etc.) of renshuu offline would require thousands of hours of work just to make, and then additional time to maintain multiple copies of renshuu (I'd then have renshuu the offline apps, and renshuu the online service) to maintain, bug fix, provide support, etc.
So, while adding offline functionality is a dream of mine, it simply isn't realistic. I still have content and functionality to add that will take me years to do, and while one might suggest that work on offline stuff in my free time (what free time?), Offline functionality for renshuu is not something that can really be done in smaller chunks - everything in renshuu is tied together, and so it may take a year or longer to have even the most basic things to test.
It's much more important for us to offer renshuu as the best we can without having to spend time worrying about price points, ad conversions, harassing users to convert to paying users, etc. So we must live within the limits that those choices create.
