In reply to ceri:
Because it's still a table based layout. This is the one thing about these forums that needs fixing ASAP IMO (sorry about the shorthand overload). since the page is generated dynamically it shouldn't be too hard to do so that it fits OK on any device.
One problem that it would introduce is that threads would become very, very long on screen. Your post would be two or three lines rather than just part of a line so you can imagine just how long some posts would get and how much vertical scrolling would be needed.