Why are there no very low bids/very high asks on GDAX? When I look at the GDAX exchange, I see that the highest ask is 8894, and the lowest bid is 8880. What I find strange about this is how close these numbers are. Suppose I were to, right now, post a bid for 5000 - something extremely low. It would probably sit on the exchange for days or weeks. So why don't I see any such bids on the exchange?

Surely somebody in the past several weeks might have gotten the idea to post a very low bid or a very high ask. Regardless of how smart or stupid an idea that is, the point is that all it takes is one person to do it in a multi-week period, and it'll show up on the exchange.

Is it just that nobody is doing this, or is it that GDAX isn't showing all of the orders posted? Or am I misunderstanding something more fundamental?
It appears that the GDAX main page only shows orders that are fairly close to the market price, probably to keep the user interface manageable.  You can see a wider range by increasing the aggregation.

You can't view the complete order book through the UI, as far as I can tell, but you can get it through the GDAX API.  If you fetch 
https://api.gdax.com/products/BTC-USD/book?level=3
you get all outstanding orders in JSON format.  There are asks as high as $9999999999 and bids as low as $0.01.