I'm doing this, in part, because it feels right to me. But also because it strikes me as more publisher- and reader-friendly.
I've noticed, however, in some of my writing groups, a certain degree of, shall we say, "writing for trilogy". (If not five or seven or however many.) I know of one who is definitely writing a big, long epic. The problem with said Big Long Epic is, in our group, what we're seeing is Book One, where Nothing Happens. Oh, there are signs and portents and prophecies of a Chosen One to fight the Coming Darkness, but no actual sense that there actually is said Darkness actually going to show up any time in the near future. The tension is further sucked out of the room by having years pass and nothing happens. There might be a great payoff in Book Three, but what reader-- and more to the point, what publisher-- is going to suffer through fifty thousand words of farm work and mysterious mentors talking about something might happen sometime, but I can't really tell you anything yet, just to get to that payoff?
But fantasy readers are, actually, kind of conditioned to expect this, and thus fantasy writers mimic this. Look at Lord of the Rings: first there's a birthday party, and then Frodo dithers about and does nothing for seventeen years, and is then told, "Hey, this ring might actually be all kinds of bad, you should do something about it." So then he launches off-- on a slow, several-month plan to pretend to move to another town... and then actually kind of does move to the other town, and then starts to go to Rivendell... only to be held up by some powerful song-and-dance man who never shows up again and DEAR GOD will the actual plot start sometime soon? No story, no urgency... and yet it is considered the godfathering work of the genre. So those elements creep into the genre, despite them being the elements of LotR that are far from laudable.