It takes a LOT longer to make a decent game, you know. Or to even announce one.
Well AC4 got announced 6 months after AC3 was released. But yeah I think it takes awhile when new XCOM game is announced.
Well, That's different, as it is an annual title with millions of dollars and tons of development teams working on it. Basically, they had people working on it for a while at the same time as AC3. Only a few games, like that and CoD, are annualized, and even then they have tons of people working to make that happen. Something which a game like this wouldn't have, as it was a it of a risk (and wasn't that big a seller anyway).
... Course, you probably knew all that and was just trying to use an exception to do the internet thing to my arguement. But yeah, most games don't have multiple teams to juggle the release.
As for the sequel, I'm guessing it's at least a year and a half away.