Typically there's a clause where the investor can be paid the cash equivalent instead of actually providing the physical metal.
If it did, mining companies would be out of business almost instantly - their product being only useful in the market when it's worthless.
Futures contracts involve actual carrying costs - storage, transporation, delivery - with real physical goods; even if you personally aren't involved in the physical aspect (by trading derivatives or the like) someone else (e.g. the people with actual silver demand for their companies) is, and they'll make sure you can't just drive the price of silver up higher by cornering the physical market.
For example : https://ibkr.info/node/992
If everyone in the market can't undertake delivery, the buyers on the other side who can will have their pick of orders, and prices will plummet.
You'll be selling heaters in Hell.
There's a bit of controversy around this.