PC case designs live for years and years. The case OEM will come up with a basic design and a couple of fascias, so you'll often get the same case with a different front and badge (even the badge size is a standard). Changes only emerge when Intel/AMD impose different cooling requirements -- you get minor reworks with extra side vents or fan mount spacing, but still the same basic design. If a white box manufacturer is buying in sufficient volume, they can specify their own fascia design on a generic case.
At work, we picked a PC supplier seven years ago and they have been able to maintain supply continuity of one basic design for all of that time. This is ideal for student labs where PCs are locked up in security enclosures that cost £100, allowing the same enclosure to be used up to three times. There is a marginal mark up for the legacy case, because newer designs are cheaper to make and provide assembly efficiencies (fewer screws etc).