Your not the only gamer in the world. Just because when you go buy a game at the store your not affected by another big budget game coming out in the near future doesn't mean that many other people aren't.
If TES V was announced right now, do you know how many gamers that are looking forward to Bethesda's upcoming published titles will immediately chose not to get those titles, in favor of getting TES V? None. Why? Well, for one, there would not be an announced release date, so there'd be no reason to wait for lord knows how long for it instead of buying games that are coming out in just a few months. Hell, even if an announcement date was released, the date would at very least
a year after the announcement of the game, as Bethesda has done for it's past games, which is not even close to "near future." If any gamer out there decided not to buy Bethesda's published titles in favor of waiting a year or more for TES V, well then they are certainly not in Bethesda's targeted market, which is, you know, actual gamers.
There is absolutely no risk in announcing TES V now. No fan that has an attention span larger than that of a fly that is looking forward to Bethesda's published titles coming out in the next few months will suddenly go "OOOOH! OOOOOOOOOOOH! SHINY SHINY NEW GAME COMIN OUT IN 2 YEARS! SCREW NEW VEGAS COMING OUT IN JUST A FEW MONTHS, I'LL WAIT YEARS FOR TES:V!!!!!!!"