At least here in the US, retailers already do better than this with their closed-loop (merchant-specific) gift cards: you pay them an activation fee to get the card and they make interest on the money until it's spent.

That also has its own unique problems - if a retailer goes into bankruptcy, the gift cards are likely to be worthless (they're considered "unsecured debt"). Buying gift cards from a brand-new startup would be taking considerable risk.

Then there's the IT integration hassles, since existing closed-loop cards are coded into their POS etc. Adding "Credits" is going to take additional development - why are retailers going to spend money to do that?

The other option would be an open-network style card, but those are typically issued (again, in the US) by companies like MasterCard and Visa & carry the usual processing fees.