This is all due to perception, and perception is governed here be the phenomenon called perspective. Objects nearby look large and those far away look small. This means that also far away distances (traveled) look smaller than the same distances near you.
A 10-meter distance 1 km away from you is relatively much much smaller than the same distance 2 m from you. A 10 m distance traveled by the train (you are in) appears much larger if you look at trees located 2 meters from the track than when you look at trees located 1 km from the track. Also 10 m distance along a line 2 m away from you is more than your eyes can even encompass, while 10 m distance along a line 1 km away is less then your eyes can discern. That's the whole secret why you can't (easily) see movement when looking far away. The relative changes are simply very tiny.
Now, if you ask why the perspective phenomenon happens at all, the answer is this picture:
As you can see, because the field of view is a triangle (it is the 3D field that we normally focus on), than the distance you can encompass with your eyes is larger the further it is from you. This also means, however, that relative distances get smaller.