This may sting some collectors/dealers here to hear this, but I would say a lot of it is about not being willing to or not worth someone's time/efforts to dig out cards. If one were organized and sorted cards by year, set and number, pulling cards like this would be a snap. If you're unorganized, it a bit more laborious to find random card of course.
When it comes to traders, I suspect most don't think it's worth their time or even the cost of postage to help another collector out unless they get something in return (what have you done for ME lately?). I have seen it more than once where Collector A actually tells Collector B they have something needed from a site or wantlist (The effort was made to seek this person out and tell them you have a need item), then A tells B never mind because B didn't have anything A could use. Now I am not suggesting A should trade a HOF/AU card for nothing because B needed it, but if A won't trade a 1995 DK common because B doesn't have any 1992 Gold cards they need, then DON'T TELL B YOU HAVE SOMETHING THEY NEED!!!
When it comes to sellers, the same excuse is valid. It's not worth the buck ten sale to pull a small handful of commons. Burbank Sports is a prime example of this type of dealer. They won't scan cards, answers questions or anything like that and fully expect us to buy their stuff. Some will, some won't. They have obviously made the call that they don't care about the small customer, only the large ones. I bought about a dozen cards from sportslots a few weeks back. Most orders were under $1 and I paid postage for each order, most of them being more than the order itself. That is the cost of seeking common stuff. I didn't complain and appreciate those sellers willing to list 18 cent cards! They probably don't make a ton of money, maybe not enough to justify the time, but they do it anyway and help keep the hobby pulsing and make it puts a few extra bucks in their pockets.
Now bear in mind I am talking (complaining) about "active" traders and sellers. For example, I may be able to hit some wantlist items from several people, but my cards are packed under my stairs. I just don't have room for them in the new house and only the stuff I am actively collecting is out and accessible, give or take a few oddball boxes. It would literally take HOURS to pull out cards and then, I am not organized to find specific singles other than perhaps by player. I am not looking to complete base sets, offering groups of cards, etc. I'd consider myself somewhat inactive for the time being, mainly buying what I need/want.
Well to that type of dealers/collectors, hopefully they receive the same effort and payoff in return that they put forth. May they not sell much and never finish their sets!
I have seen a fair number of people on this site offer and even send free of charge some of these "insignificant" wants to other collectors, just to help someone out. We have members giving away superstar autographs in contests, people donating cards to a charitable cause and other random acts of kindness so there are plenty of people out there who will help for the sake of helping and the spirit of collecting. They don't always have what you need, but when they do, that type of person is there to help. We just have to take the good with the bad and hope we don't run into ugly!