The only thing that comes with a retail drive is, lets see, an instruction sheet that no one ever reads, and a disk with software you can download anywhere, and a pretty box. I don't know about you, but the last 2 or 3 drives I bought did not have any cables in the box. All of my cables came with my motherboard. And the 1 or 2 times they did come with cables, they were those really stiff cheap ones that I tossed into the trash anyway.
So I ask you, why would most people want to pay more for a retail boxed drive?
You only need that driver if your using an ancient OS and have never used a SATA drive before. In most of the cases the driver is superfluous. And in the few cases it's not you can get it online almost as easily as you can from the CD.
Newegg makes more more profit on an OEM drive than a retail version.
Others have pointed out the differencies in contents.
OEM drives take up less storage space as an oem drive takes up about a third of the space as a retail package, this save on shipping costs and storage space. These cost can be considerable when dealing in the 1000's of units. At the manuf there is the cost savings of packaging. Newegg (just using as a vender) can pass half of these costs reductions on and pocket the remainder, kind of a win-win situation. As pointed out the vast majority do not need the "extras" anyway.
There also use to be a diff in warrranty, but pretty much any more this is a non-issue.
I've seen an interesting comment that's worthy of mention in this thread. Retail packages are larger because they contain cushioning for the drive. Each retail package has it's own cushioning as supplied at the factory.
OEM drives come in large shipping cartons containing a dozen or more drives. - once they're removed from the carton and sold individually it's up to the seller to provide cushioning for each individual drive during shipping. So with OEM drives you're relying on the seller to package the drive safely for transport.
Not a problem with some, could be a concern with others...