It's in part because advertising still drives visibility which drives sales, and all the main discovery mediums are centralized and sell ad placements to the hi

It's in part because advertising still drives visibility which drives sales, and all the main discovery mediums are centralized and sell ad placements to the highest bidder. Combine that with the typical algorithmic promotion algorithm, where Amazon etc want to show you things they think you're likely to buy, so they mostly sell you things that are already top sellers.