Because it all goes back to this idea that Obi-Wan meant "died" when his actual line was "was destroyed." Granted, Luke does say "then my father is truly dead" to Vader later on in the movie, so take that for what you will. In any event, Obi-Wan was neither destroyed nor killed until ANH and that's why he's still Alec Guinness but Anakin is now Hayden. It's really yet another reason for why it was a stupid change.