Why aren't scallops sold in the shell?

I'm curious, why isn't it possible to find scallops sold in their shells at the fishmonger or supermarket? It seems like it would be a convenient way to store and transport them, not to mention that the shells would provide a natural barrier against contaminants. So, what's the reason behind this practice? Is it related to food safety, ease of preparation, or some other factor that I'm not aware of? I'd love to hear your thoughts on this matter.