The heat added to the gas causes the molecule in the cylinder to move in random directions, but only the molecule that moves in the direction of the piston can exert a force on it.
The molecule that moves toward the piston can be converted into work.
The added heat would have to be converted into work in order for the gas molecule to move.
This is very unlikely in a large group of molecules.
A group of monkeys who are hitting typewriter keys at random and who by chance type out the complete works of Shakespeare without error are the odds against the complete conversion of 1 cal of heat into work.