Why doesn't controlled impedance depend on track length?
The characteristic impedance of a transmission line is not the same thing as a lumped resistance, it just happens to have same units. Similarly, certain amplifiers are designed to have a current as input and a voltage as output, so their gain is a ratio of volts to amps, with units of ohms. But that doesn't mean those amplifiers have much at all in common with resistors.

The characteristic impedance of a transmission line is the ratio of the voltage and current of a wave that can travel along the line without distortion. If you tried to inject a signal with a different ratio of voltage to current, you'd find that part of the injected signal travels one way on the line and the other part travels the other way --- you'd create a reflection. 

Since this property of the line --- the type of wave that can travel along it without distortion --- is specified by a ratio of voltage to current, we can give it a value in ohms, and call it an "impedance". But just like the gain of a current-to-voltage amplifier, that doesn't mean it has any other behavior in common with a resistor, and you shouldn't expect it to.