BINs automatically relist, so it makes sense as a seller to put them up and just let them go round and round. Listing is a pain in the ****. Also, competitive bidding is virtually non-existent, so why sell as an auction and only get the opening bid price, assuming it gets even one bid? You basically have to set the opening price at a level you would be willing to sell at given eBay take a fee for setting a reserve.
With regard to the number of items listed on eBay, they long passed the level at which it was possible to view most of the offerings. 15-20 years ago the average number of listings in the Britsh section was around the 3000 mark with a significant percentage selling. As I write, this number stands at 424,780, which further reinforces the rationale for listing as a BIN - eventually someone will view it, and one day someone might even buy it! What is certain is that relatively few people will see it when listed for 7 days, but furthermore be willing to bid it up. You need a few interested parties to generate a good price.
Given the advances in computers, storage, processing speed and general all round capability, it is only likely to provide facilities for an even greater number of listings. If the capacity is there it will be used.