Why all java methods are implicitly overridable?

In C++, I have to explicitly specify 'virtual' keyword to make a member function 'overridable', as there involves an overhead of creating virtual tables and vpointers, when a member function is made overridable (so every member function is implicitly not overridable for performance reasons).

It also allows a member function to be hidden (if not overridden) when a subclass provides a separate implementation with the same name and signature.

The same technique is used in C# as well. I am wondering why Java waved away from this behavior and made every method overridable by default and provided the ability to disable overriding behavior on explicit use of 'final' keyword.