There are quite a few reasons, I warrant - first and foremeost, because no one likes to pay more money but get less in return. 

For eBayers who have been around for a while, like myself, we've watched the standard FVF rise from ~5% (which got lower the higher the item price was) to a flat 9.9.%, which then extended to postage costs. For some sellers, it's still acceptable and managable, for others it meant their FVF was increased to a point where selling on eBay was no longer viable, so being cheaper than other forms of selling is a moot point for them.

Fees have more than doubled for some sellers, and it's also now variable for many of them (because of the FVF on postage, which for some items means a big difference depending on where the item is being sent to), which makes it very difficult to account for in a fair way. 

These increases have brought few tangible benefits for sellers, there are some features, sure, some of them very welcome, and I'm still here, selling my stuff, but while they've caused less impact on me and I don't spend a great deal of time complaining about the fees and changes, I fully empathise and understand how / why it's all gotten a bit much for others.

Stealing is very much an issue online sellers have to deal with - some eBay / PayPal policies facilitate it just a little bit too easily. I also have to pay for electricity to run my business equipment.