Because in this case the value can change overnight, and Valve doesn't otherwise have control over the market (value) of the items.
Expensive clothes aren't much more expensive to produce than cheaper ones, but it is known that brands like Louis Vuitton will destroy old stock rather than put it on sale. Some products are intrinsically valuable though, because production can't scale as well - some cars (although a brand can do a new design and mass produce it), watches, semiconductor manufacturing, etc are constrained due to their complexity.
Expensive clothes aren't much more expensive to produce than cheaper ones, but it is known that brands like Louis Vuitton will destroy old stock rather than put it on sale. Some products are intrinsically valuable though, because production can't scale as well - some cars (although a brand can do a new design and mass produce it), watches, semiconductor manufacturing, etc are constrained due to their complexity.