Licensing. "OEM" technically isn't legal to sell to consumers and OEMs generally don't sell current versions of software through other parties. Like, you may find an old OEM version of Paintshop that was bundled with PCs, that the OEMs sold off in bulk when a new version was released, but you wouldn't find the latest OEM version. The software companies allow it to be sold to individuals on the gray market to get another user.
There is no longer an "OEM" version of Windows on discs, since the days when they stopped including a reinstall disc with PCs, but rather a "System Builder" version on disc which people refer to as OEM which is supposed to be sold only to people building and selling white box PCs. But similar to the OEM software, Microsoft allows it to be sold to just anybody on the gray market because it means another Windows PC in the world.
Both OEM and System Builder software licenses are tied to the machine they originally were activated on. According to the terms, you can't transfer it to a new PC, with the vendors having their own ill-defined explanations of what constitutes a new PC. Like, you can replace a failed motherboard or CPU with the same or comparable, but you can't replace it with an upgraded board, and of course building a whole new system and transferring it is out, but installing an upgraded GPU or more/better RAM does not count as a new machine. Hard drive replacements no longer count as new PCs either, I think. Again, the vendors are not very stringent about ensuring this doesn't happen, but there are systems in place to make it harder now that everything requires activation. You may need to go through a phone call or online interrogation and have to say that you're just reinstalling on the same machine or had to replace a failed component. (Since Microsoft and others use hardware IDs to match your license key to your hardware, they could easily tell that you have replaced multiple major components but may not be able to tell that it was an upgrade.)
Retail licenses give you the right to uninstall and move it to another machine as often as you like, though you may sometimes have to go through the same interrogation to claim that you have removed the original installation and are claiming your right to move it.
The actual software on the DVD or USB key, System Builder or retail, is identical, in the case of Windows, although often far out of date compared to a current ISO download. The downloadable ISO can be used to install using any type of key.
You can buy gray market license keys from innumerable online stores for like $35 or even less. They are licensed Microsoft retailers and use loopholes in licensing to provide keys that aren't TOTALLY adherent to the terms, and for which you don't actually get any rights to support for Microsoft for example, but they almost always work just fine and are indistinguishable from full-priced keys as far as the user is concerned. Occasionally you'll get one that will be unable to activate, Microsoft will say it's already been registered/activated, and then the seller will need to send you a replacement.
