There’s a lot of nuance to the answer and I’m not a nuclear engineer, just an EE and occasional nuclear enthusiast. The biggest issue is the cost-effectiveness of reprocessing the “waste”, which you are completely correct about still having a ton of residual energy available, back into a useable fuel for a sustained nuclear chain reaction.

Some of the fission products that are produced in reactors are actively harmful for sustaining a chain reaction (neutron poisons), Xenon-135 being a prime example. Xenon-135 only has a half-life of about 9 hours (which means it’s pretty spicy) but it also has a massive neutron capture cross-section. If it doesn’t capture a neutron, it emits a beta particle (electron/positron), which doesn’t contribute to sustaining the reaction; if it does capture a neutron it becomes Xenon-136 which is pretty stable. In both cases, it’s sitting in the fuel but either useless (yay) or actively hurting the neutron economy (boo).

At some point in the future it might be economically advantageous to reprocess “used once” nuclear waste and use it in a second cycle but for now it’s way cheaper to get more fresh uranium and process that into fuel instead.