E.g. there is a plugin which highlights double-click-selected text in the editor, but I always asked why this is not a core feature because I think it is very helpful for many coders.
Because it costs time and cpu to do this, especially on large files. And not everyone wants it, I for one find it hugely annoying the way Eclipse highlights the [expletive deleted] out of everything.
Also purely textual highlighting in code is only useful in some simple languages like C [ducks @b4n volley] whereas more sophisticated languages with overloading and scopes its highly misleading.
Its possibly useful in text of course, so its in a plugin.