It's the way it is due to gameplay. Can you imagine the outrage if one single player could satisfy the demand and crash the prices of the entire bubble with only 1 ship load?
They should certainly be harder to
obtain in that case to justify the scarcity of demand.
But there certainly are some expensive non-mined commodities - Palladium [1], Muon Imager, all the "regional" ones like Reinforced Mounting Plate - where collective player action can quite easily - though unintentionally - remove all the supply, and has in some cases.
It makes Colonia trade CGs quite interesting as the CG target is 1.2 million for Tier 8, and the local stock + a week's production refill of three named commodities is often also about 1.2 million. In the bubble it just means you have to go further for it, of course.
There's definitely a balance to be struck between allowing interesting things to happen like regional - or even bubble-wide - supply or demand shortages ... and making sure that at least some trade goods just keep moving so beginning traders aren't constantly losing money or unable to fill their T-6s. Frontier seem to be working on adding depth to the trade system slowly and carefully to get that balance.
[1] Yes, I know this can also be mined. But that's a much slower process in normal cases.
