Because you are measuring at a very broad and basic level.
Steve is more productive than Susan.
Great. How much more productive? Can you turn it into a number?
Can you still do it consistently when Steve and Susan are in different teams in different parts of the organisation trying to achieve different goals?
I've done DB upgrades that took 10 minutes and I've done DB upgrades that took 3-4 months. What changed was not my productivity but the nature of the problem. Yet from the outside they were both just DB upgrades.
If Susan had done the DB upgrade in 12 weeks could we confidently claim that Steve could have done it in 11 weeks? Steve hasn't even done a DB upgrade since he joined the company. Perhaps Steve could have done it in 10 minutes?