Because ctags has extremely limited context sensitivity - i.e. it has no concept of the kind of symbol it's indexing, so editors can't use it in different ways

Because ctags has extremely limited context sensitivity - i.e. it has no concept of the kind of symbol it's indexing, so editors can't use it in different ways depending on the cursor point. If you look at the code for IntelliJ plugins, the code for doing different sorts of completion in different contexts is actually really complex. Edit: moved unrelated questions to a top-level comment.