Not sure what you are asking for here. Issue is listed as P3 and is being actively worked on. That’s all the information I have for you at this time regarding this specific feature.
“Chrome” does not get “copied into” Brave. Both Chrome and Brave run on the same underlying engine (Chromium engine — as do Edge, Opera and several other browsers). What this means is that instead of Brave (as well as the other browsers mentioned above) needing to build and maintain it’s own engine from scratch, we instead leverage a powerful existing engine, then [heavily] modify it to create something new.
Further, there aren’t many things that are done “easily” with respect to software/browser development on this scale. We’re talking about literal millions of lines of code and millions of potential interactions. On top of that, there’s the design considerations of the feature like the copy being used, the way it looks, etc., then there’s a testing process as well. There is no "press this button to make the feature appear and work perfectly " option.
So to answer your question, what’s “stopping this” — which actually means “why is it taking so long?” — its because there is a good amount of work required, on top of all the other work our developers have. I hope it helps knowing that the team is aware that this feature is missing and are actively working to implement it. You may follow the progress of this feature by keeping an eye on the issue linked above.