For one thing, they hold their value. Even if you use it you can expect to get more than half your money if you resell it. What are some things that hold their value or increase like that even when you use them? Guns? Jewelry? Anyway, not many things - think about how the value of a vehicle drops by thousands the second you drive off in it.
Also, I never personally have had to use my knife to do anything but cut, but the Strider folders were originally designed and adopted for hard military use where they might have to do things a knife isn't really designed to do, or at least isn't the optimal tool to use.
The SMF definitely has an official NSN number and although I would not want to debate how SOCOM chooses their gear or what is "official", from everything I have read the Strider SMF was the only officially adopted folding knife , except maybe for Emersons. I think that the military versions were made to higher standards though, at least on paper.
So everything together- the collector resale value, the military designation, the unique look, and the fact it is "overbuilt" give a buyer confidence to spend more money to purchase one.