Because it's is hard for him. Very hard.
His rational mind is not mature yet to predict the future. His brain can not assist him to see beyond a certain point.
Without the rational mind he does not see that the effort he has to put into learning a difficult task will pay him off later. He wants a cause and effect now.
If his action does not give him satisfaction in the moment he will try something else.
So he chooses a very smart rout.
Even though adults have their rational mind developed well, adults don't act that much different either...
We all adapt differently. Some of us try walking with a different gait pattern. Some of us try locomotion with a wheelchair.
Some of us have bodies that can not move according to our will at all.
Our adaption also depends on our personalities.
Some of us are thinkers. Some of us are doers. And anything in between.
Thinkers will adapt by giving up the physical aspects and escaping into the dream world.
Doers will adapt by pushing their bodies to extremes.
And anything in between.
When we work with our children on the physical aspects of their existence without taking into consideration their personality they will find a way to avoid that movemet.