Actually, it's not just about lists, it's about how variables work in general.
It'll probably be more obvious to you with this example:
If Snap! did what you suggest, then we'd end up with two copies of the sprite, which clearly doesn't make sense.
A variable is not a box where you put a unique thing, but just a name or a label you give to a value. The same value can be labelled hundreds of times, and it will always remain separate from the label.