There are already more complex, deeper games on Facebook, but they are orders of magnitude less popular than these simple Zynga games.
If you go deeper, you go niche.
BTW it's similar for game themes (independently of game mechanics) - more mainstream you go, more potential players you can get.
Just check Facebook games charts. They are dominated by down-to-earth topics - farms, restaurants, aquariums, pets, etc.
I'm counter-example myself. I actually reverted from being hardcore game player to playing mostly simple web games.
And I sure know others which went this way - from AAA titles to Zynga.
Here I said it. Most "real" gamers would be probably too ashamed to admit it :).
And about growth - I'm not sure what is the cause and what is the effect. In non US regions games themselves could actually be the driving mechanism for Facebook growth:
One thing I've been coming to terms with- just because something makes no sense long term doesn't mean that it's not a valuable course to pursue short term.
If, and it's a big if, Zynga can reinvent itself in the next 6-9 months, then this initial phase was brilliant in that it gave them the resources they needed to move on to real domination. If they don't reinvent themselves, a few people will have made a lot of money, but nothing permanent will have been built. A real opportunity will have been missed.
But they can't keep on the same path, it just won't work a lot longer.