Because eBay is doing a Donald Trump, and putting eBay's interests above all else.
I think we can all see that eBay was forced by competition to drop fees from private sellers. To make up for this they added an entirely contrived "buyer protection fee" which basically didn't give buyers any protection that they wouldn't have had anyway, but was in effect a buyer's premium.
I don't know what eBay has with courier services and Royal Mail, but I think we can reasonably assume that eBay receives some commission for all the delivery services sales they arrange.
I'm not taking eBay's side, anything but, but realistically eBay is in business to make money. Take a look at eBay's share price. It's well on the way up again, so it's shareholders must like what it's doing.