Because if you buy a game and immediately sell it you are flipping it. Bought a few games (some cheap, some at market rate), got them home and within a day or two knew it was not for me. Sometimes I genuinely bought it to flip. A few times I needed the cash because a game I wanted more came up for sale.

Because people think that they can negotiate you down based on it.

Because I don't want everyone to know what I have in my collection. It isn't what it would cost me because its taken a lot of good deals to work my way up, but its still a chunk of change.