Defense is an afterthought

Why is it that when Greatest of All Time lists are constructed, defense is an afterthought. Oh, it is nice to list it, but it is not essential; any defense displayed is just gravy. The exceptions are Ozzie Smith in baseball and Bill Russell in basketball. Both were decent offensively, but it was on defense that they made their name. Both are extremely rare cases. Usually offense is all that matters and all that is considered.