More broadly: a text input box is in fact a small but surprisingly comprehensive text editor. It supports a cursor with insert, delete, and overstrike; highlighting, undo/redo, cut/copy/paste; and shortcut keys for all that. It supports every keyboard layout and every input method. It obeys standardized focus rules. It's even got word wrap and spell checkers these days.
So, you want to go your own way? How much of that do you need to reimplement? And how confident are you that the subset you choose doesn't completely ignore some vital use case you forgot about?
You certainly can't get away with just wiring keystrokes to a field.
