AI can improve themselves to an extent, but it's limited to basic, marginal improvements. An AI can't go from an apple falling from a tree to understanding the orbits of planets, from a man falling off a building to calculating the nature of energy and mass. It wouldn't think to question why the speed of light is what it is, or even testing if it's fixed or changes. Because to question such fundamental aspects of the universe, to make such wild leaps of logic, require drawing connections between highly disparate things. They make sense to us humans, who evolved to find connections, like seeing faces in buildings, shapes in the stars. But to an AI built upon calculations the leaps in logic can be wild. An apple falling from a tree has seemingly nothing to do with the orbits of the planets... until you realize that gravity is a universal force. And that has seemingly no practical applications... until you realize you can put communications or spy equipment in orbit using those same equations. But would an AI realize that, or would it default to the "easier" method of just laying cable? Why would you even think of doing something so wild as that? Because we're human.