Series endings

It appears today that every first book is but the prelude to another, which in turn leads to still another. All books, particularly YA fantasy, seem to come in threes. Why is this? Is the world-making that fantasy demands just too much for one volume? Or does saving the world or universe by one person require more volumes than one book?
I have no objections to series, but I do feel that when a book comes to the last page, it should have an end, even if a sequel is possible. Too many fantasy novels today end with nothing resolved, with every thread left untied and the hero about to embark on some new, vital journey with no finish in sight.
Of course, that neatly ties into keeping the book-buying going. Read the first in a series, love it, and the market is there to buy more. Unfortunately, the second volumes rarely solve anything. Instead, the issues and problems mushroom and then the reader has to get the third volume. Is this a con game? Do I really believe that the author could not have solved the problems the hero faced in one book? Yes, he or she could have.
My own writings involve sequels. But I hope I have always tried to tie up loose ends and reached a conclusion at the end of each story. One does not have to read the sequel to "Beetle." It is just a better story if one does. But my novel "Red Patch" definitely ends. Green Shana, the sequel, mostly involves Leenea's daughter, a different country and an entirely different set of problems and challenges. That is the kind of sequel I like.