They literally aren't diamond. They contain no carbon. Because they're zirconia, not diamond. They're marketed as an alternative to diamond.

How do CZ gemstones compare to diamond gemstones visually? It looks like CZ has lower refraction index but higher color dispersion vs. diamond, so that seems like it would result in an attractive gemstone. The main disadvantages seems to be slightly lower refractive index (so less internal reflection and brilliance?), lower hardness (disadvantage for the gem getting scratches - possible advantage for not scratching your sapphire glass watch, or smartphone camera lens?!) and higher weight.

edit: seems that moissanite (silicon carbide, perhaps unsurprisingly) is another diamond-like gemstone that is actually harder than sapphire (though hexagonal crystals vs. [cubic] diamond or CZ), and less prone to fracture than diamond

To everyone except those with a trained eye, they're virtually indistinguishable; however, because CZ isn't as hard, they tend to scratch and dull over time and thus lose their luster.

If we're talking about 'quality', CZ just doesn't compare for longevity. That said, unless you're talking about a daily-wearing-for-many-years piece of jewelry (i.e., an engagement ring), CZ is just fine for most folks, especially considering the cost difference.