Speaking from my experience as a trader from Localbitcoins, the main reason is probably because it is intrinsically difficult for the support to solve any dispute. The problem with gift card is that the seller could give the trader an used gift card and try to pass it off as legitimate. Due to the structure of the various sites, it is not possible for the gift card to be tracked and thus it is difficult to see who is telling the truth. It's more likely due to the hassle of managing such issues rather than its legitimacy.  I've bought tons of gift card from there for personal use and I've never experienced any problems after redeeming them.