Lets look at a few reasons.
The first application for such an operating system is as an educational tool. Without a collection of reverse engineering tools, it is impossible to examine MS-DOS in any great detail. Reverse engineering MS-DOS may also break any existing licensing agreements between you and Microsoft, so check your license before applying a disassembler to it. My guess is that MS-DOS source code is almost entirely in assembly language and the kernel itself guaranteed to be assembly language. Even if you did see the source, it would be more difficult to follow than if coded in a high level language.