Question

Why do most 3D scanners have an optimal scanning distance?

Answer 1

This is all about the accuracy .

3D scanning is the process of analyzing a real-world object to collect data on its shape and colours .
The collected data can then be used to construct 3D models.
The purpose of a 3D scanner is usually to create a 3D model.
This 3D model consists of a point cloud of geometric samples on the surface of the subject. 
These points can then be used to extrapolate the shape of the subject called reconstruction process . 
If colour information is collected at each point, then the colours on the surface of the subject can also be determined.

Because it collect the data at each point so it should be more accurate.

Accuracy depend on the distance or range of the scanner . 

Taking an example :

The time-of-flight 3D laser scanner and Triangulation based  3D scanner both are an active scanner that uses laser light to probe the subject

But Time-of-flight and triangulation range finders each have strengths and weaknesses that make them suitable for different situations. 
The advantage of time-of-flight range finders is that they are capable of operating over  long distances(in kilometres) . 
These scanners are suitable for scanning large structures like buildings.
The disadvantage of time-of-flight range finders is their accuracy. 
Due to the high speed of light, timing the round-trip time is difficult and the accuracy of the distance measurement is relatively low, on the order of millimetres.
Triangulation range finders are exactly the opposite. They have a limited range of some meters, but their accuracy is relatively high. 
The accuracy of triangulation range finders is on the order of tens of micrometers.
So maintaining the accuracy maximum 3D scanner use optimal scanning distance.

Here i have explained the points for the 3D scanner .