Why can't linux run dos programs?

Because it is an entirely different KIND of OS. It is Unix-based (as are the BSD family, whose FreeBSD is used in Mac OS X (which is therefore also part of the family), Sun Solaris, BeOS/Haiku (to a degree), etc.), so it's derived from a system that was developed back in 1969 (and because of it, its family is very stable and well-documented, while not at all obsolete thanks to frequent releases and bugfixes, at least that's the case with Linux, Mac OS, and the *BSDs (thanks to a completely different philosophy of programming)). Unix is also the birthplace of C, the programming language. Linux hails back to 1991, 4 years before the first usable (but still crappy) Windows was released - back then, Windows was treated as a necessary evil and in fact was even a poor system for gaming thanks to hieroglyphic or nonexistent DirectX. True, you'd have to use a hex editor to install it back then, but those versions of Linux began with a 0. Linux and Windows are entirely different in construction and are a world apart. When Linus Torvalds wrote it, he didn't want a Windows clone - he wanted a kernel which could carry a home-friendly Unix-like OS (he liked MINIX very much, but he didn't have the type of computer right for it). And incorporating Windows code as part of a distro (Linux is just the kernel (a part of the OS that handles memory sharing and multitasking), a distribution is a complete system with programs, like Ubuntu or Fedora) is out of the question, as Microsoft does not give its source out - if it did, everybody would compile and fix Windows himself. And finally, since the early nineties, many Windows programs, beginning with Notepad, ending with Dishonored and Deus Ex: Human Revolution, can be run through Wine, which is a collection of libraries that substitute the ones Windows programs call out to (programmer-speak for "attempt to use") - at the base, an open-source clone of Windows. As Windows hasn't changed much since NT - they just added new apps and graphical shells - Wine remains very up-to-date.

Therefore, thanks to Wine, Linux, Mac OS X and Free BSD can into space/run DOS+Graphical Shell (what Windows really is) games and programs.

EDIT : Oh, there's a reason for it not being programmed in. Keep in mind that Linux is supposed to be a small kernel, not overloaded with things like compatibility layers.

And RMS would shoot himself if he found out Linux (which he hates because people don't pronounce it "Ghnoo plus Linux") included an easy way to gain the freedom to run proprietary software.

Which are needed because...

You don't have Windows libraries on Linux, as you already said. They are incompatible because Windows libs often call out to the registry, etc. and are differently written, making porting useless, except for maybe generic libraries, like stdio in C, etc.

On top of that, the systems delimit lines (of code) differently - pure Unix, BSD, and Linux appends a Line Feed character to the end of every line, Mac uses the ancient Carriage Return, and Windows uses both - so if you recompiled the source code, it would be useless, especially with languages that don't use semicolons as forced End-Of-Line. Nowadays the preferred newline character can be selected in an editor, but how are you going to convert the source code of a Win32 program, most of which are proprietary? That would require decompilation, and if the author found out about an unauthorized port, he'd sue your sorry behind. So no DOS for anyone, except maybe a "hello world" program whose author unfortunately selected Line Feed in his editor and can be chmod +x'ed.

THE API and ABI are different.

They are so different and DOS execuables are so old that emulating them is the easiest thing to do.

In addition adding the DOS ABI into the kernel would be difficult and make changes and improvements very difficult, in return for very little gain.

The short answer is that they CAN be run under linux.

www.dosbox.com

You will be amazed at how many old DOS programs run just fine using DosBox under Linux.

Because DOS is something that window's runs. .exe files are command's that the window's OS parses through. And that language is built into the Operating system. Linux is'nt window's, and the .exe file "language" is'nt built into the OS.

It's like if you take a guitar and try to make it sound like a violin. It can be done, but you still get bonus points if you can do it because it's not easy.

Use Dosbox.

Disk Operating System (a.k.a. Dirty Operating System) is mechanically designed different from UNIX and Linux.

BECAUSE IT IS NOT DOS