Air defense is not static. Even fixed launchers can be moved, and reacting to how your enemy is operating is an important part of air defense tactics. The famous F-117 shootdown happened because the air defense operators carefully planned around how the US was using its aircraft. If most Iranian air defenses were destroyed in the first few days, it'd make more sense for them to hold whatever was still available for the sort of situation where they had much higher chances of scoring a kill than just throwing it out there to get destroyed immediately and accomplish nothing.