The reactors we use now can't run on depleted fuel. It's true that like 90% of the uranium is still present in deleted fuel but that's not the problem. The problem is the build up of fission products. The fuel itself is essentially a ceramic pellet in a metal tube. As it gets "burned" some of the atoms in the fuel split into new smaller atoms. Specifically some that are "poisons" and some that are gases. The poisons absorb neutrons much more easily than the fuel atoms, stopping the chain reaction. And the gases create pressure inside the fuel pellet. If enough gases build up this can cause the pellet to crack, releasing them into the metal tube. Now you have one less barrier to releasing radioactive material and your pellet isn't in the shape it's supposed to be anymore making it harder to know how it will react.

So we can't use them in current reactors, what about "low power" reactors? This is a problem of economics. Depleted fuel is hot, but not hot enough to quickly boil water and make steam. It's like asking why don't we power our house off all the free heat coming off a person all the time. The temperature difference and heat output is just too low to be useful in any but the smallest niche application.

So how do we deal with the depleted fuel? We reprocess it. Break down the fuel and dissolve it in acid so you can recover all the useful uranium to make new fuel. The leftover radioactive material can then be turned into glass and safely stored or you could feed it into a different type of reactor that "burns" the waste turning into something that only needs stored for 200 years instead of 20,000 years. All this has been well known and understood since the 80s but politics consistently gets in the way of actually doing anything.

Indeed, this is like the techbro approach to nuclear reactors, which seems like the worst of all possible worlds: all buzzwords and bullshit hiding barely-concealed scams about shit that can absolutely kill you.