Remember when AIB prices were lower than reference prices?.
Unless the cooler was utter garbage or the custom PCB was made of horrible parts (see: some bottom of the barrel Sapphire and MSI models of the past), non-reference AIB cards have almost always been more expensive than the pure reference models. AMD and Nvidia reference set ups have always been on the bottom rung of prices for GPUs. Since everything is just bought from either company and the AIB slaps their brand on it vs designing a custom cooling solution or an entire custom PCB its not only cheaper but also they're competing with every other AIB doing the same thing so its advantageous to stick with the MSRP. In the past there were simply more cards at, or near, MSRP than there are now, but custom AIB cards were still more expensive than MSRP.