it’s not like Israel doesn’t have plenty of more effective conventional ways to kill people
All modern militaries have other effective ways to kill people.
However, killing civillians on purpose and maiming them with deep open wounds that are resource-intensive to treat is normally seen as a war crime, and a weapon that is openly designed for that would raise even more international condemnation.
This is probably the reason we repeatedly see white phosphorus used against civilians in conflicts where permanent territorial occupation is a goal (other examples by Russia in Ukraine, by Indonesia in West Papua). It has a terrifying, catastrophic effect on members of the population, and there is (im)plausible deniability.
The excuse of using it as a "smokescreen" really does act as, well, a smokescreen.