This is mostly thanks to volunteers creating mods/renderers/etc, which in turn is only possible thanks to Epic's focus on user-generated content through the Unreal editor. 15 years later, and people are still adding to the game[1][2].

[1] http://www.globalunreal.com/forums/viewtopic.php?f=64&t=1511 [2] http://www.ut99.org/viewtopic.php?f=5&t=5347&p=58359#p58359