Why missile fuel rarely doubles as a selectable warhead

Could a missile use the same material as fuel and warhead, giving more blast at short range and more range at long distance?

The idea has been studied, but residual fuel is more often an incidental part of the explosion than a practical selectable warhead.

The concept would trade unused propellant for a larger short-range explosion and reserve that fuel for range on distant shots. Designers have examined the idea but have not made it practically viable. Missile fuel may already add fire and blast after impact without functioning as a controlled warhead. Exocet damage illustrated how residual fuel could intensify the effects of a hit. Turning that incidental contribution into a predictable, selectable yield introduces design and safety problems beyond merely carrying extra propellant.