> Why would that be the practical limit for intelligent extraterrestrial life?
Might not be, but it really doesn't matter that much. The problem is that most of the fuel expenditure is going to lift fuel, not rocket or payload. WIth even the most energetic chemical fuels, only 15% of the total takeoff weight is rocket+payload, and improving the weight of the rocket doesn't change that - you just get whatever weight you trim off the rocket materials as potential payload, and the returns diminish quickly.
Might not be, but it really doesn't matter that much. The problem is that most of the fuel expenditure is going to lift fuel, not rocket or payload. WIth even the most energetic chemical fuels, only 15% of the total takeoff weight is rocket+payload, and improving the weight of the rocket doesn't change that - you just get whatever weight you trim off the rocket materials as potential payload, and the returns diminish quickly.