The plug resistance comes almost totally from a resistor inserted in the copper electrode. The resistor’s only purpose is to eliminate RFI, high frequency noise emitted by the spark discharge characteristics, which could disrupt the workings of the PCM, ABS computer, etc. I suspect NGK decided to use a standard 5,000 ohm value for most of their plugs. That would be based on their evaluation of what suppresses RFI noise from their spark plugs. The resistor has little, if any effect on the engine performance.

The total resistance of the ignition circuit has three components - 1) the wire resistance; 2) the plug resistance; and 3) the air gap resistance. The air gap resistance significantly exceeds the sum of the wire and plug resistance. That's why it can take 20,000 Volts to jump the gap (depending on many factors, unrelated to spark plug resistance). Unless you are dealing with an older capacitive discharge ignition, the plug resistance is not important with regard to ignition system performance. It's only there to reduce RFI.