Because Entity Info is designed primarily to be an information display - not an edit window? And SketchUp has added additional function of changing the geometry from there when it can figure out what you want to do? And when you select an edge and change the length, SketchUp has no clue whether to add/subtract length for one end, the other end, or split the difference?
If, instead of selecting an edge, you selected a face, then tried to change the area, what would you expect SketchUp to do? Guess?
Based on this, the fact that the entity window can be used to change the length of an edge at all is, in my opinion, a bug!
It’s nice that, in some cases, it is possible to make changes with the entity window. Cases such as:
- Assigned layer of selected geometry
- Radius and # of sides of an arc or circle (as long as SketchUp still identifies the selection AS an arc or circle)
- Name (in the case of components, both definition and instance names), description, IFC code
See a pattern here? You can change things that don’t affect the geometry, or where SketchUp doesn’t have to guess how you want to change the geometry.