They are not real diamonds
These companies use misleading words to try their hardest to convince you that they sell real diamonds instead of what they are actually selling, simulated diamonds, which in itself is a fancy term for the material Cubic Zirconia. So why are these companies trying so hard to mislead their customers? Unfortunately, sustained deception-based marketing is vital for their business model to survive. They will liberally apply phrases and descriptions throughout their sites and promotional materials specific to real diamonds like 'eternal' or 'sparkling', which don't apply to cubic zirconia.
The real value of cubic zirconia stones becomes evident, which is next to nothing, which many people, unfortunately, find out when trying to resell. The setting itself might carry some value, while the cubic zirconia, like any other diamond simulants, is always discarded.