Machines capable of manufacturing exactly the same component time after time, with exactly the resistance you want, would be very expensive, and so would the products they turn out. A resistor would cost a dollar instead of a few pennies. The machine itself, and its output, work within tolerances.
The cheapest mass-produced resistors are guaranteed to be within 20% above or below the resistance marked on them. And you know what ? For most bench- work and prototyping, that's usually close enough.
