In MRI the Hash class is core and written in C. In particular, methods are programmed with explicit C function calls and polymorphism is not honored.
For example, = is crucial in this implementation to maintain the @keys array but hash.c invokes rb_hash_aset() originally. This prevents method reuse through inheritance and forces us to reimplement stuff.
For instance, we cannot use the inherited #merge! because albeit the algorithm itself would work, our = is not being called at all by the C code.
# File activesupport/lib/active_support/ordered_hash.rb, line 37 def initialize(*args, &block) super @keys =  end