[Melkman]

The reason you can not build a 0% tolerance resistor is the laws of physics. A very high precision resistor can certainly be build but it will never be perfect. Increasing precision has a cost to it. For normal resistors the shape and thickness of the film of resisting material is calculated and the tolerance is mainly dictated by the precision of the manufacturing process. Increasing this precision of the process adds cost. High precision resistors can be trimmed to specification. When you manufacture the resistor with a lower resistance you can use a laser to trim some of the resistive material away. This is an extra step and adds extra cost. While this can be very precise you are limited to what you can measure and there is a limit to that. Also precision is limited by environmental factors like heat, humidity and aging.

[adql]

That's also how many higher end analog chips are made, just blast it with laser till it fits tolerance. Making a bunch of chips in less precise process then trimming them chip by chip ends up cheaper than going to more expensive processes.

[YetAnotherNick]

We are saying the same thing. For most applications, 5% tolerance is fine if could be cheaper. But for some applications it is worth the extra cost.

[klodolph]

It sounded like you were saying that a 0 tolerance resistor would be worth the extra cost, and the reply is pointing out that a 0 tolerance resistor is not actually possible anyway, and as you approach 0, the cost increases beyond whatever your limit is.