Free operating systemsEdit
First, no. Windows and Macs aren't the only OSes out there. There's things like Linux, ReactOS and OpenBSD that are free to use and modify. Second, Windows and Macs are common.
Free OSes, Patents and DRMEdit
Patents allow non-free software to profit off of algorithms that are protected under the law. DRM will gain money to the big corporations. Unfortunately, patents and DRM damage the popularity of free software.
Free OSes and DVDsEdit
When we buy DVDs, they are encrypted and use DRM. But there's decrypters. Most are again, non-free and use patented algorithms. There are few free DVD decrypters that are open-source. One such decrypter is DeCSS, used in VLC to view DVDs.
Free OSes and SoftwareEdit
Here we go...
Linux was a kernel replacement of Minix and today GNU/Linux (the Linux kernel with GNU userland) runs the server empire. Too many people think that Linux is for so-called "hackers" and "programmers". This is why Linux doesn't have good software. But Wine is a project to fill in the gap between Windows and Linux. It doesn't have all of Win32 ported.
ReactOS is replacement for Windows. The reason why so much software doesn't work with ReactOS is APIs. ReactOS wants to make a free version of Windows NT/2000/XP/2003/Vista. But most software used with ReactOS are software that supports like say Windows 95/98.
The BSD OS is not used much. That's why not much software is able to run on *BSDs.