Because text rendering depends completely on the system. It might look how you want on a Mac or on Linux, but not on a PC, so if you fudge the display co-ordinates it will break it on other platforms that otherwise looked OK. It's also the kind of thing that can change with OS updates, user settings, or browser updates. It is better to not rely on text being pixel perfect, or if you do need it, make the text an image in a sprite object.