Why is walking not considered exercising?

Most people walk at around 1 to 2 miles per hr.This is very slow movement of your legs

At this pace, it does very little in the way of exercising any muscle groups. But it is better than sitting in a couch all day.

If you were to speed up your pace to a reasonable fast 5 mph . then you would be moving virtually all muscle groups. Then you would be doing a fast walk exercise.. And yes, it would be then classed as an exercise, so long as it is at least one mile a few times weekly.. And only then you would see and feel positive results