Most german sedans are tested to ridiculous standards that they cannot hit. For example, the VW phaeton was requested by Ferdinand Piech to go 186 mph while being stable and maintaining a good AC temperature. This is despite the fact that it is limited to 155 mph.
I believe the limitation to 168 mph is therefore of power, and critically tires. Tires are likely the main factor, as you need special tires when you are going above around 170 mph. This is for example why the Rivian R1T is limited to around 115(?) mph as its tires could not handle a higher speed, even if the powertrain can. With Lucid's low resistance and low profile tyres, I suspect the effect is exacerbated causing it to be limited to 168 for tire safety reasons. In addition, testing at higher speeds just makes sure that the car is aerodynamically sound, so even if a customer does the stupidest sh*t ever by limiting the remover and going full send stock, it can handle it(apart from the tyres, which will probably blow at 180).
https://www.tiresplus.com/tires/tire-buying-guide/tire-speed-rating/ this supports my argument as if you scroll down, "W" rated tyres are limited to 168 mph. Could somebody check if anything is mentioned of a speed rating on the tires?
To me, having 168 is already very impressive, knowing that it can handle well in excess of that speed and STILL be stable and considering its low rolling resistance, low profile tires. As soon as I get a license, I would definitely love to see how silky smooth the Air is at these speeds on a runway.
TL;DR: Most important reason is tyres.