Because cybercriminals know people search for "offline installer." They wrap malware into fake installers. The official method (ISO + DISM) is the only secure route.

Technically, the "offline installer" for Windows 10 and 11 is already in your hands if you have the Windows installation media (ISO or USB). You don’t need to download a third-party .exe ; you simply need to point Windows to its own source files.

Windows Update will download the necessary files and install them.

This indicates a Secure Boot or BIOS issue.

If you are in an environment without internet access, the most reliable method for standard Windows 10 or 11 versions is using the Deployment Image Servicing and Management (DISM) tool and your original Windows installation media. Microsoft Learn Install .NET Framework 3.5 on Windows 11 - Microsoft Learn