^^^ Too much complacency in tennis. At the top, it pays fairly good money, so there's no real incentive to drive their way to the very top. This goes for both the men's and women's tours.
If the Williams' didn't get bored and/or injured, the WTA would be "The Venus and Serena Show" and I don't think there'd be any woman motivated to deter them other than maybe Sharapova and to a lesser extent Jankovic. Opponents have gotten used to losing to Serena so much they just don't build up that motivation factor necessary to exact revenge. That and they know they get $$$ regardless of the outcome, even more if they're cute.
"The two-hander is just a more secure ball." - Navratilova
Dunlop 4D 300/M-Fil 400/Aerogel 300. Babolat PS. 2HBH/Cheap Players club's CEO