"That process" being manual modeling? Sometime it's simply time restraints. When you're on set, you have a very small amount of time to get all the data you need. For example, you might only have the time to take reference photos of an actor, since a talented modeler can use them to build a model. It is much harder to turn it around, and derive textures from take scan data. (Not to mention, you would probably miss a lot of details) It could also be that the scan data was botched in some way. There are many variables to it.

I've built many 3d scanners in the past, and am in the process of writing yet another. (See https://simplymaya.com/forum/showthread.php?t=36965 for the latest) No acquisition technique is 100% effective. It's a constant trade-off between resolution, speed, portability, and robustness to certain BRDFs (material properties).

For example, some techniques are fast, but low resolution. Others are slow, but produce a huge amount of resolution. Some, like lasers, are dangerous to humans; while others are simply too large/heavy to bring to set. And let's not forget that different scanners exploit different optical properties. For example, some are more robust to specular objects, while others are more robust to projector defocus, etc.

Plus, no matter what scanning technique you use, there is no guarantee that it will be "correct". It takes a human to make it right. I'm all for technology and automation; hell, I'm a TD; but at the end of the day, software is stupid. Humans need to make sure the job's done right