The problem with White Phosphorus is that it is technically an incendiary weapon, not a chemical weapon. The reason being is that it is a multi-purpose substance in combat. It's used for signalling, smokescreens and illumination. This technicality prevents it from being banned as it causes severe and excruciating burns. It literally melts flesh and it's almost impossible to put out.
It is a crime to use it as an attack weapon, but it's multi-purpose use gives the users of this substance plausible deniability. They can claim that they were simply illuminating the battle field and any casualties/injuries to civilians was completely accidental.
I'm sure militaries take advantage of this.