Question

Why do less active Americans not increase their activity levels

Answer 1

Several reasons:
-they don't want to
-they think they don't need to
-"don't have time"
-too lazy to make time
-too hard
-don't like to
-don't fell the need for it
-don't see the benefits of it
-has better things to do
-too busy
-doesn't care
-hasn't found the right kind of exercise for them > doesn't like to, or too hard (etc)
-forgets to

Answer 2

Less active Americans could possibly not increase their activity levels because, it can be of multiple reasons. They could be too lazy, they don't have much time on their hands, they're way too busy, or there could be something personal going on in their life to be active. They can be stressed. There are more examples than these to why Americans who aren't active increase their activity levels.