A multi purpose missile or projectile would be needlessly expensive, complex and very likely less effective trying to do too many things.
Anti-air requires speed, maneuverability and typically can disable aircraft by proximity detonation in a certain volume of space.
Anti-tank is usually anti-armor. The energy has to be directed very specifically (or the thing has to be huge) to be effective against modern armor.
It would be difficult to make both happen with a single type of warhead.
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