Roblox itself isnt open source yet, so it wouldnt really make much sense if Sober open sourced itself. The Roblox client is still closed source. Also:
Why is Sober Closed-Source?
We made it closed source to reduce the potential for abuse (which would lead to Roblox blocking us again). Additionally, distributing Sober as a Flatpak also allows us to implement tighter restrictions in order to prevent any abuse.
Hope this helps out :)
Some more info about Sober:
How does Sober work? Is it official? Will it ever get blocked by Roblox?
Roblox blocked Sober during the Hunt, so it is most probable that it MAY BE blocked in the future. For now, just hang on folks...