This forces me to ask, who has played every race/class combination in the game and used every monster in the manual in the less than two years the game has been out?
What's this demand for new and more driven by at the table.
Because not all the material in the core books (and now the SCAG) is interesting to everyone. If I'm not interested in being a caster, there aren't that many options in the PHB for example (barbarian, non-Eldritch Knight fighter, monk, non-Arcane Trickster rogue). In addition, people miss some of the options available in older editions (for me, that list mostly consists of Martial Adepts a la Tome of Battle, psionics, and a proper treatment of Eberron mechanical material).
True, but other than the Core books, all the 5e stuff has been done under licence. They've pretty much just licenced out the Tabletop RPG, but on a book-by-book basis rather than all at once.
They haven't been done under license. They've been outsourced, which is a completely different thing, and freelancers aren't exactly a new thing in RPGs.
Onyx Path Publishing has a license from White Wolf. They think up things to do within that license, and check with their licensor if that thing is OK. When they're done, they also check with the licensor if what they've made match their expectations, and finally publish it. The book is sold by Onyx Path Publishing, who presumably pay White Wolf for the license.
Wizards of the Coast outsources production of certain books to outside companies. Wizards thinks up a thing they want done, and then pays another company to do that thing. That company then delivers the agreed-upon product to Wizards. Wizards polish things up (I'm not sure how much of the editing/layout stage and such is done at Wizards and how much is done by the production company), and then publish it. The book is sold by Wizards of the Coast, who have presumably already paid the production studio for their work (though there may be royalties or something involved - I find it unlikely, but ultimately the exact contract is none of my business).
If the adventures/sourcebooks were done under license, even a book-by-book license, they would be sold by the production company instead of by Wizards of the Coast. This is pretty much how it works with the miniatures and spell cards, for example.