Number of drinks produces different blood alcohol concentrations, and therefore different degrees of intoxication, in different size people.
It's the same reason why having someone urinate in your coffee is worse than having them urinate in your ocean.
Besides, if a driver is stopped for erratic driving on the highway, it's a lot easier to determine his blood alcohol concentration than how many drinks he's had (and how long ago) (and the size and alcohol content of the drinks).
