Heavy clipboard users like myself might similarly ask "why are programmers allowed to use things like the IPv4 control in place of ordinary text boxes?" You know, the kind that support both copy _and_ paste! This would also reduce the temptation to design "auto-advance" edit boxes that lose focus on keystrokes other than tab, frustrating for a number of reasons.
One of the nicer things about OS X applications is that, for whatever reason, third-party Mac developers seem less prone to "superfluous" custom control creation, so, among other things, OS-level support for things like UI automation, dictionary look-up, and Emacs key bindings in edit controls actually work reasonably well.