Well, it's really part of the "marketing" strategy that most retailers (internet or otherwise) use. You do a significant mark up on an item, so that when you do a sale later on, pyschologically to the customer, it looks like a good deal because it's now XX% off the original price? There will be that category of customer who wants immediate gratification and is willing to pay the initial premium and the rest probably wait for the discount.
Not every person has access to an HErmes store or a good relationship with an SA (esp in Asia) to land a birkin. So (reputable) resellers remain a viable option for many.
The thing also with hermes birkin/kelly is that each bag looks different to another - they are all handmade by craftsmen and the leather used for each bag is different. Personally, I have passed on bags that were in the color and leather I wanted because that particular bag (construction and grains) did not appeal to me KWIM? I have noticed that people are willing to pay quite a significant premium for a basic colored birkin via a reseller simply because that particular bag appealed to them.