Why doesn't MS-DOS initialize the DS and ES registers?

Question

Why does the initialization of the DS and ES registers has to be done manually by the programmer?

For example:

MOV AX, DTSEG
MOV DS, AX

On the other hand, the CS and SS registers are initialized by the operating system (in MS-DOS). Why is this so?

Answer 1

Because CS and SS registers are essential for program execution in contrast to DS and ES registers which point to user-defined data segments. By default no data is present in the executing program this nothing to initialize the DS and ES with. As a program writer you can specify where your data is by setting the data segments registers.

Edit: as was correctly noted by @FrankKotler, in .com file (the entire program size doesn't exceed single segment), DS and ES are initialized and equal to CS. For other execution models, DS and ES are initialized by pointer to PSP (which isn't the pointer to real user data).