Why is “whomse” not a word?

I often hear people say something like

For whose benefit is that?

Should it not be

For whomse benefit is that

Who -> Whom
Whose -> Whomse

I know “whomse” is not a real word. My question is: why doesn’t it exist?

Answer

The easiest way to think about this is to compare to he him his:

Who gets the benefit? He gets the benefit.

To whom does the benefit accrue? The benefit accrues to him.

For whose benefit is that? That is for his benefit.

For whomse benefit is that? That is for hims benefit.

Obviously that last is unnecessary/wrong—in place of hims (or him’s) we have his, and in place of whomse (or whom’s) we have whose. (Also, sound aside, whose is no more related to who’s than his is to he’s.)

That’s the quick-and-dirty, functional answer; it’s also accurate that whose and whom evolved alongside each other, subject to different influences than what might make sense from our modern English point of view. From the OED Online:

whose, pron. Etymology: Middle English hwās, later hwǭs, whǭs, altered form of
hwas, hwes, Old English hwæs (< **χwasa*) genitive of hwá and hwæt,
through the influence of hwā, hwǭ who pron., hwām, hwǭm whom pron.
(Later Middle English whas probably represents an unstressed variant.)

—”whose, pron.” OED Online. Oxford University Press, June 2016. Web. 20 August 2016.

Attribution
Source : Link , Question Author : Albert Renshaw , Answer Author : 1006a