Why is it so difficult to build a non-Microsoft OS that runs Windows software natively?

Windows applications are built specifically for Windows. They expect particular Windows APIs, system libraries, file paths, registry behavior, graphics interfaces, security features, and sometimes undocumented quirks. A compatible operating system would need to reproduce all of those behaviors accurately, including the bugs that applications may rely on.

Projects such as ReactOS are attempting essentially this: creating a free operating system compatible with Windows software. The challenge is that Windows is enormous and constantly changing, while ReactOS has far fewer developers and cannot simply use Microsoft’s proprietary source code. Matching the full API, driver model, hardware support, and application behavior is an extremely long-term project.

It isn’t simply that Microsoft blocks every other operating system. The main issue is that Windows software is usually closed-source and depends on a very large, complicated platform. Reimplementing the documented interfaces is difficult enough; matching undocumented behavior, security systems, multimedia stacks, graphics features, and commercial applications such as Office is much harder. At some point, running Windows itself in a virtual machine is often more practical than recreating all of it.

Wine and Proton take a different approach by translating Windows API calls into Linux and other platform calls instead of being Windows themselves. That works surprisingly well for many programs and games, but compatibility is still incomplete. Applications that depend on unusual APIs, copy protection, proprietary drivers, kernel components, specific versions of Microsoft libraries, or tightly integrated services can still fail.