Someone said it earlier...supply and demand. Most folks prefer to buy their fish small and raise them themselves. Not just fish, but any pet. Unless they're not getting as a pet, but as breeder stock, there probably is LESS demand for a larger fish than for a smaller, younger one. Again, same with any pet.
The larger fish are more difficult to house...hence why people often sell them after they've had them for a while, especially when they don't know what they were getting into. Which makes the supply greater than the demand...reducing the value of what you're asking about.
The folks who know what they're getting into, and want them that big, typically don't want to take the risk that the fish wasn't well cared for and could be less healthy than if they raised it themselves...so again, less demand for the older, larger fish over the younger/smaller.
Buying these kinds of fish is NOT an investment...it's a hobby or a passion.
Hope that makes it a bit clearer.