The answer is simple
Take a perfume
go to an open area
press the spray button for 2sec in air
its obvious that you cannot get the smell of the perfume after a minute
now spray the perfume on four dress at a spot for 2 seconds
smell it after a minute, the intensity of the smell is obviously more.
The Same happens in case of your doubt
The urea and smelly waste products are produced in our body but is distributed almost uniformly in our whole body that is the intensity is low
but when oru kidney filter this waste materials and produces urine, the concentration of this smelly urea is high in urine and hence it becomes smelly.