Do you apply the same reasoning to aircraft and insist on planes that haven't been test flown?
The effects of wear and tear can be measured, quantified, and predicted based on real world performance. Issues that happen on startup are most likely to happen on first startup, and a vehicle that has already flown can be expected to have far fewer such issues.
An old claim that's looking rather foolish at this point. SpaceX has managed to reduce production costs to levels their competitors can't match, and still sees immediate benefits in cost from booster reuse, in addition to the logistical benefits allowing much higher launch cadence. China, Russia, and the EU are all working on reusable systems in an attempt to catch up.