Governments can cause oversupply, governments can cause undersupply. Bet you can think of plenty of examples without my help of government policy resulting in supply for some goods decreasing or increasing relative to demand.
Governments can do a lot. Like, they can pretty much dictate what you're allowed to make, how much they're gonna tax you on profits from making something, who you're allowed to sell it to - they can absolutely decide to cause an asset to appreciate.