Same reason people are mean on the Internet
There's a local chocolate shop that has lots of turnover because they hire college girls to staff them most of the time. But the owner is on top of it, keeping good records, and about two weeks before Valentine's Day, they call me and say, Devilbunny, this is so-and-so from Chocolate Shop By You, you bought some chocolate-covered strawberries last year, would you like to order again this year? Yes, here's my credit card, and I'll pick them up after 3 pm on the 13th. Thank you, see you then. I walk up to the window, pick them up, bring them home, and say happy Valentine's. It's really the only thing she wants (I get her flowers but not roses - why be so cheesy when every other flower is like, half-price?) specifically. They make it easy to spend money with them instead of someone else. I don't have to wait more than a minute - they have a dedicated window for prepaid orders. That's service, and it's worth the small premium.
In B2B, it's about a relationship and trust. My FIL sells clothes. He's the middleman between the manufacturers and the stores. He has a territory and based on what's happening in other stores in the region he can steer them toward the right stuff for their store. I.e., what's this store's target age range, how affluent is the area, and so on. In return, when he does a good job with them, they will learn to trust his advice on what will generally sell well, and he ends up getting better commissions. He sells about a dozen lines from about six manufacturers, though about three or four of the lines tend to make up the bulk of his income.
Since he's very good at his job, he can demand higher commissions than other salesmen just to take a line on. He's got the on-the-ground relationships, and a manufacturer will give him a bigger cut because he's not going to have canceled orders, returns, or headaches for them.