Because their bags are full and need to quickly make space and don't wan't to leave stuff on the ground. And since blues and greens are so common they have no extra value.You still make more money putting in on the trading post then pulling it down later to vendor than you do leaving a high level blue or green on the ground.
Maybe he plays multiplier games to play with people not compete against them. The GW2 community in game seems to very nice. I would largely say this is due to no mods and tools which allow people to compare and put down others.
I tried guild wars at launch but only played a few hours because it didn't catch my interest. Tried again a few months before GW2 was being launched and still found it boring. Been playing GW2 about every day since launch. So I guess I was part of the larger audience they where going for.
I think they had to reinvent the wheel to get enough money to make it worth the trouble. A true sequel would not of excited all the people that didn't want to or like to play GW.
99% of the guilds will take part in PvE and 10% if lucky will take part in GvG. Which one do you think keep more people playing and possibly buying stuff from the cash shop?