Why doesn't the kidney just absorb urea without performing ultrafiltration first then selective reabsorption?

Human kidneys perform ultra-filtration then selective reabsorption before producing urine. However, this seem rather redundant, for water and vital salts are pumped out into the nephron, then absorbed again. The kidney could have been more efficient by choosing to single out urea and other waste products immediately when blood enters the kidney. Why is it not so then?