What is the difference between Windows 10, 11 OEM key and Retail key? Why is OEM key so cheap?

There are 3 differences between Windows 10, 11 OEM and Retail:

1. The Windows Retail license can be transferred to another PC while the Windows OEM license cannot, because it is already locked to one PC.
2. Windows OEM versions allow all hardware upgrades except for an upgrade to a different model motherboard. As for the Windows 10 Retail license, it will not be affected by any hardware replacement.
3. An OEM Windows 10 license is much cheaper than a Windows 10 Retail license. (The cheapest of the versions is Volume license, but often encounter errors during activation)

Aside from those differences, there is nothing different between an OEM key and a retail key.