A lot of Unrealers do play offline, they have since the original Unreal Tournament. Very difficult to tell how many (majority definately seems like a stretch), but release a mod without decent bot support and they pop out of the woodwork.
The reason is simple - the UT series has always had quality bot AI and the multiplayer landscape is simply getting more and more annoying to play with. Ah, the good old days when it was just the occasional neo nazi racist bastard you had to deal with, and not a twelve year old singing like cookie monster into his microphone.