Why do sellers assume that the buyer should contact them first to settle a dispute. Isn't that what the dispute resolution process is for?

The dispute resolution centre should only be used as a last resort. It is the same as if you have a problem with something you purchase from a B&M store - you go back to the store first and if you have no luck you can then go to an authority such as Consumer Affairs for help.

Ebay recommend communicating first with your trading partner if there is a problem. If the problem can't be resolved then a dispute can be opened.

Also when a buyer goes to leave non positive feedback a pop up box appears suggesting the buyer contact the seller before leaving feedback. It even brings up the link to contact the seller to make it easy for buyers.

Ebay consider the key to successful transactions on eBay as direct communication between buyers and sellers.

http://pages.ebay.com.au/help/buy/communicate.html