Why is the president's power to convene and dismiss congress limited?

Well he can't convene it whenever he sees fit or he could do it all the time and cause problems. He can convene it in times of trouble when it's necessary. For example when starting a war because only the congress can start a war. He also can't dismiss it when he feels like it, rather he can do it only when he has valid reasons to do it, or he could dismiss them whenever they caused him troubles.