That's not a stockpile, they have to extract and refine it to use it. Russia understood it the hard way with the refineries attacks.
And no, oil is more expensive (especially nowadays) to extract than uranium.
There's a reason nobody ever became rich with a uranium mine, all the value is in the plant and the market price barely covers extracting it, some mines even closed because of the price being too low.
We're not talking about ore, rather fuel. Uranium ore might be inexpensive, I don't know, but converting that ore into fuel is not an easy task. I'm guessing that the reason it's not profitable is that so little of it is actually needed, relative to oil.
And no, oil is more expensive (especially nowadays) to extract than uranium.
There's a reason nobody ever became rich with a uranium mine, all the value is in the plant and the market price barely covers extracting it, some mines even closed because of the price being too low.