Metal models photograph very very badly when
raw. That is why companies like
http://www.hfminis.co.uk put a very slight wash on their bare metals before photographing them. People on eBay aren't doing this.
The majority of flakey painty metals on eBay are done by people who're selling off collections or dealers, and they care very little for the minis, which were likely done by someone without intimate knowledge of the painting process. Someone who spent months or years painting their army to a high standard is not going usually to bang it on eBay, the internet's bargain bin.
There's very little discernible difference in quality between good metal and good plastic for most miniatures.
The limitation of metal is minimum thickness, not its ability to hold detail.