If you're skilled at modeling, you can do a lot to customize figures.

But IMO, it's more than just polygon count. DAZ's figures aren't necessarily the biggest. Rather, it's the textures, and the hair, and the clothes, etc. There's tons of those for DAZ's figures, free and for sale, and that makes a big difference in usability. Most artists don't render a lot of images of naked, bald women. (And hair and clothing are probably the most difficult and time-consuming things to model, so even people who can do it themselves may prefer to use pre-made items sometimes.)

(If you do render mostly naked women...well, there are models out there that are, more, ah, "anatomically correct" than Vicky.... ;-)