Of course, he could still escape from the invisible wall by activating his Metacs to increase his strength greatly and breaking through the wall in his way by force. But it will cost him his lifespan to do so.
On the other hand, it won't cost the world anything important to trap him. After all, the wall was created with the power of the world, not the power of a Metac.
So rather than spend his lifespan breaking through walls, he decided to spend his lifespan beating the dragons. That way, every amount of lifespan that he burns beating the dragons will cost the world a lot more lifespan to heal.
It is a case of being active or reactive. He had been forced by the world to fight the dragons. But now that he has exposed his strength and gained some initiative, he is unwilling to back off so easily.
The will of the world became desperate when he refused to let the two dragons go. It began creating a force field around him to bind and suppress him.
