The terminal guidance systems use broad-spectrum imagers to identify warheads and select their point of impact. Radar is only used to point them in the general direction.
Against this type of terminal guidance, a decoy must have the same shape, size, orientation, and spectral signature of a warhead, and maintain these properties while being ablated. As a matter of engineering, a decoy with these properties will necessarily have mass and volume costs that approach that of a warhead, with none of the benefits of a warhead.
From a weapon design perspective, effective decoys significantly reduce the range of the missiles they are attached to. That is not a tradeoff any weapon designers are willing to make because range is one of the most critical attributes of an effective weapon.
Even if the problem of an effective decoy ballistic warhead could be solved, it will a still quite costly to get it "there" at "that" speed. Might as well fill it with high explosives.
Against this type of terminal guidance, a decoy must have the same shape, size, orientation, and spectral signature of a warhead, and maintain these properties while being ablated. As a matter of engineering, a decoy with these properties will necessarily have mass and volume costs that approach that of a warhead, with none of the benefits of a warhead.
From a weapon design perspective, effective decoys significantly reduce the range of the missiles they are attached to. That is not a tradeoff any weapon designers are willing to make because range is one of the most critical attributes of an effective weapon.