> This is a Python specific problem caused by everything being boxed

I would say it is part python being highly dynamic and part C++ being full of undefined behavior.

A c++ compiler will only optimize member access if it can prove that the member isn't overwritten in the same thread. Compatible pointers, opaque method calls, ... the list of reasons why that optimization can fail is near endless, C even added the restrict keyword because just having write access to two pointers of compatible types can force the compiler to reload values constantly. In python anything is a function call to some unknown code and any function could get access to any variable on the stack (manipulating python stack frames is fun).

Then there is the fun thing the C++ compiler gets up to with varibles that are modified by different threads, while(!done) turning into while(true) because you didn't tell the compiler that done needs to be threadsafe is always fun.

What is going on here is not, that an attribute might be changed concurrently and the interpreter can't optimize the access. That is also a consideration. But the major issue is that an attribute doesn't really refer to a single thing at all, but instead means whatever object is returned by a function call that implements a string lookup. __getattr__ is not an implementation detail of the language, but something that an object can implement how it wants to, just like __len__ or __gt__. It's part of the object behaviour, not part of the static interface. This is a fundamental design goal of the Python language.