And the primary reason for that is longitudinal stability.
Pitch excursion (due turbulence, say) down, speed increases. Downward "lift" on tailplane/elevator increases. Mainplane lift increases. Plane pitches up.
Hah, your post made me go off and do a little more research in aircraft stability, which in turn led me to another pprune post that I learned 2 new things from! Maybe I should post that in the "can we learn anything new here" thread! See, us noobs take a lot from this place, even though we may not be able to contribute...
Anyway, I shall go away and study now, I have hijacked this thread enough. Thanks folks for the pointers.