I have noticed people selling there bitcoin to resellers who will buy low and sell high making few profits from what they bought from others.
In addition to what others have said as I believe your question most have been answered due to the type of users I see who have already commented above, to add, there are other reason like not trusting the exchange. I have had users contact me with thousands of dollars in Bitcoin to trade, that's they sell to me. They fully understand that I won't have such funds to credit them like instantly at hand but still they're willing to wait on me to do whatever I'll do to give them their funds which usually results to me making use of exchanges but they don't trust those exchange to use themselves probably due to past ugly experience.
Also I have had those that just want an instant transaction, exchange normally requires atleast one confirmation to display your deposit and additional confirmation to be able to process your transaction but with peer2peer, if you trust the user, you can process a zero confirmation transaction.
This isn't recommended to carryout with users you don't have personal relationship with as a scammer can take advantage of you by double spending and you lose your money without receiving the Bitcoin. And just to stressed what others have said, it's just about preference and don't stress yourself over what others are gaining, focus on yourself.