Because adding an 'out' selector completely changes the entire philosophy of how rules have always been managed. It doesn't just add a feature. Its presence will make people think they need to use it, when they really don't, even if it's hidden.
Fixing the Floating rules list to better signify things like the interfaces being used is more feasible.
Making "out" easier to use is a bad idea. For most people, it doesn't do what they think it will do, and just makes things more complicated than they need to be.
Yes, it can make certain specific things easier, but for those cases, Floating rules work. If you find Floating rules deficient for that case, fix Floating rules.