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 :)