You’ll probably say: because that’s already implemented on the web UI.
Sure, it already is. But, I’d reconsider the idea to implement a native dark mode within the full Android (not sure if iOS as well) app itself. That white glowing interface hurts my eyes. T.T
The answer to any “Why no …” question is almost always “because it’s work, and no-one has had the time/desire/idea/knowledge to do it yet”. You probably don’t intend it this way but to me it comes across as somewhat rude, questioning why we haven’t given you something you think you deserve.