We can't force nuclei to decay, but we can make them undergo reactions that turn them into other nuclei which decay faster.
There is some promise of doing this with waste from nuclear reactors, so that we don't have to store it as long.
Uranium is not the problematic part of nuclear waste.
The problematic part comes from elements that are produced during reactor operation, either as fission products or as uranium nuclei that caught neutrons and then decayed to other elements.
Others have already said we can't influence the rate of decay, and this is true. The decay rate is an intrinsic property of any given nucleus.
But that wasn't really your question. You asked why we can't keep decaying the uranium until it isn't radioactive anymore.
The answer is that we can, but it takes a bit more than just leaving the fuel in the reactor.
The stuff that goes into the reactor has uranium in it, but it isn't pure uranium; most of it is U238, which isn't very radioactive at all.
Theres also other stuff in and around the fuel like the moderator made out of heavy water or graphite that slows neutrons down.
In order to start and sustain a fission reaction, you need a high enough density of neutrons with just the right energy level to split another nucleus and generate more neutrons. We get that by carefully balancing how many neutrons get produced with how many neutrons get absorbed.
With fresh fuel thus is straight forward. As it reacts it builds up all sorts of other decay products that absorb neutrons and poison the reaction. These decay products are still very radioactive, they just don't produce the right kind of neutrons.
So if you want to keep reacting the uranium you need to reprocess the fuel to get rid of the waste products. It turns out that this is extremely expensive and dangerous to do. So much so that most folks just mine fresh uranium out of the ground instead. Unless you have other uses for the waste, like bombs.
Most sane folks don't want more nuclear bombs around than there already are, and the kind of buildings and machinery you would use to reprocess fuel for power are exactly the same ones you would use to build bombs (this is what is meant by dual use technology).
So if you don't want anyone to have a legitimate reason to have that kind of equipment lying around, you make sure the world price of uranium is just low enough to ensure it's easier to get new fuel instead of reprocessing the old fuel.
So what you can do with fission products is "transmute" them with neutron bombardments. There are complexities to it that make it difficult in practice. In principle, what you can do is bombard the radiactive waste with neutrons, this makes the nucleus MORE unstable so that it is more radioactive. It then decays and now you have a stable daughter product.
As an example, say we have strontium-90 with a troublesome 28 year half life. Well, if you hit it with a neutron, it become strontium-91 with a 9.5 hour half life, which becomes yttrium-91 with a 58.5 day half life, which becomes stable zirconium-91. This would shorten how long you have to look after the waste.
Same thing for Cesium-137 with a 30 year half life. If you get it to absorb a neutron, it becomes cesium-138 which has a half life of 32 minutes and becomes stable barium-138.
Both of those examples have to do with some pretty "bad actors" as it comes to rad waste storage in the first few hundred years.
The longer term decay products that are millions of years half life are generally transuranics and can be fissioned to become shorter lived fission products so they go from millions of years to tens of years (I'm simplifying a bit here).
The issue with this is a lot of fission products don't have large neutron capture cross sections. Even then, if you did irradiate them, not all of them would absorb a neutron. Some of them would absorb multiple neutrons and may turn into a more problematic nuclide than you started with (say, already stable fission products that you just now made radioactive through neutron activation). You also have a hard time treating the original spent fuel to separate out the specific species you want to transmute.
I believe some folks have advocated accelerator driven transmutation as a possible source to break rad waste down more quickly it's largely plagued with the same issues as neutron bombardment.
The bottom line is, we don't actually have that much spent nuclear fuel, concrete casks are relatively cheap, pretty effective, and the longer you wait, the easier it generally is to recycle/reprocess the spent fuel.
Are actually asking "Why don't we just fission all the uranium until there is no more?"
The reason why this does not work is that the results of nuclear fission result in things that are radioactive.
Additionally the chain reaction that is used to fission uranium is not flawless. Neutrons are easily absorbed by impurities in the uranium and sometimes the uranium itself. So as more and more of the uranium turns into other byproducts, there are more atoms around to absorb the free neutrons.
So that is why we can't just fission away all the radioactive materials on the earth. Most are unfissionable and the growing byproducts of uranium make it harder and harder for chain reaction to keep going.
That's kinda what we're doing. Nuclear plant worker here. When the fission process slows down enough to where it isn't producing the heat required to make sufficient power the rod bundle is removed from the reactor during an outage. When it comes out it is under water and stays there because it is still highly radioactive. They place the rod bundles in a cooling pool where it will sit for around the next 20 years. Until it is cool enough and stable and safe enough to remove. Then it goes into those concrete coffins where it sits for the rest of its life until the world finds a decent way of truly disposing of it.
I think this is a conceptual issue. The way you're starting the issue makes it sound like you think we control the fission reaction. We don't. There reaction happens according to probability, and we have nothing to do with it. It happens in the deposits of material still in the ground, it happens in the waste stockpiles, it happens wherever an isotope exists.
What we do in nuclear power it's harness the energy that nature provides.
There is no way to know how long it will take for someone somewhere to invent something as revolutionary as a rapid radiation decay process. But if your interested in the subject look into learning more about college level physics. There are lots of fascinating things to know there.