Some authors can write and publish book after book after book in rapid succession, using the same time-honored process over and over. Romance novelists often come to mind, as their books are often formulaic.
Yes, some books are fairly easy to write. Perhaps you get the first draft down in three months, revise for three months, let it sit for a month, let your critique group review it for two months, then rewrite based on those comments for the next three months until you have something that’s in fairly good shape.
But that nice year-long process that worked so well for Book A just doesn’t work for Book B, which demands more time for some reason. Maybe the characters are new to you, and you have to get to know them, or perhaps you can’t get past a certain plot point. Either way, your formula isn’t working, and you get discouraged.
I don’t think all of a writer’s books can (or should) be written using the same process over and over. That’s not to say that the authors who use the same process for multiple books are wrong — perhaps they’re just lucky to have books that conform like obedient children. Also, published authors have the pressure of deadlines that might force them to stick to a certain process, and if that process doesn’t work, the author may be forced to give up on one book and start another that will hopefully work out better.
Books are like kids… they don’t all grow up the same way.