> I can equally ask why keyboard dead keys aren't typed after the character.

That does have an answer: keyboards predate electronics. A mechanical typewriter key press slams a type bar into the ribbon, and on the way back, catches a mechanism that releases the carriage (which is pulled along by a spring) for one column. A dead key is very simple; it just omits the catch. A postfix dead key would be more complicated, since it would have to add an early backspace action, and harder to press, since backspacing pulls against the carriage spring.