Because ASI is 'superior to the best human' while AGI is 'matches or surpasses humans'. It's hard to agree on what 'matches humans' actually means. Would matching humans in coding involve the level of the average human (basically no coding ability), the level of a junior (the largest group of professionals), a senior ('someone competent') or of the best human we can find?

Or as the scene from 'I, Robot' goes: Will Smith asks the android: 'Can a robot write a symphony? Can a robot turn a blank canvas into a masterpiece?' and the android simply answers 'Can you?' ASI sidesteps that completely