So can oil. Energy security is an important priority for a global power.
Stockpiles are good. Own supply chains are better.
Sure. That doesn't remove stockpiles' inherent disadvantages: finiteness and vulnerability. Relying on uranium stockpiles would immediately put China at a known limit in a war of attrition that wouldn't constrain their adversaries.
Reserves != stockpiles.
Nothing can compete with the energy density of uranium.
Oil is relatively easily, inexpensively, and quickly mined and refined. Compared to, say, uranium.
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.
Uranium is better for Chinese energy security than oil. But this still leaves China at Moscow's mercy. That's not too differet, energywise, than the situation is now.
https://wits.worldbank.org/trade/comtrade/en/country/CHN/yea...
Uranium is abundant, but not homogenously so [1]. (China has some. But not a lot. And it's bound up expensively. And it's by their population centres.)
For the Americas, Europe, Australia, southern Africa and Eastern Mediterranean, burning uranium makes sense. For China, it trades the Strait of Malacca for dependence on Russia and Central Asia.
[1] https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1800.pdf