What I don't understand is why the free software movement so often reinvents the wheel, Telegram releases all the code to their UI but any new security focused chat app always starts from the ground up.
I think developers prefer writing code they understand over trying to understand someone else's code. I've taken a look at Telegram's codebase, and replacing the Telegram API with the Matrix API is way harder than it sounds. Telegram wasn't developed with this flexibility in mind, and the Matrix SDK has its own way of doing things.