Because eBay want to keep their profits up even though their market share of e-commerce is falling.

That's not to say there are better alternatives for sellers (although I am sure there is for some), but that there are now many alternatives for buyers.

The one thing a site like eBay has going for it is that you don't need any particular qualifications to sell (some sites are specialised, others require applications and are subject to approval), and almost all of the legwork, in terms of attracting general traffic to the site itself, is done for you. For some sellers, that's worth paying for, even if it invokes a few grumbles along the way.