I think it´s because the average quality is still pretty high at DAZ. You don´t have to tweak the figures a lot to make them look good, just load them an pose them and hit the render button in most cases the render will turn out somehow good.
Try this with the built in content, the G2 characters ( except for Kelvin) look somehow freakish especially Simon and Sydney they just suck , Judy needs a lot of morphing on her face alone to get a decent result, the P6 kids have a real bad rigging, just try to pose their fingers, I could go on for a long time.
DPHoadley did a great job with the picture he posted here, but he spent an awful lot of time to morph, remap and pose the figures. Most people just don´t want to spent that amount of time and DAZ offers them a lot of easy and ready to use content. If you join the Platinum Club you have more than 2000 products for less than 2$ sometimes even less than 1$ to chose from.
I personally don´t like every DAZ Figure and really hate V3 and M3 but they did a pretty good job with M4, I really love him. Not so sure about the new kids yet and not a big fan of V4 but she´s a great improvement over V3.