Because currently fresh uranium is cheaper than reprocessing spent fuel. Further, to really close the fuel cycle and get the full benefits of reprocessing you need breeder reactors, another technology that exists and is proven to work, but with current Uranium prices isn't yet competitive with thermal reactors and a once-through fuel cycle.

None of the cost estimates on this subject really make sense because of the period of time involved.

For instance "no nukes" think it is a scandal that we could spend $100 billion on Yucca mountain, but the average nuclear plant makes about $500 million per year in electricity so that is about 2 years of energy production. It's significant, but it's not crazy.

A reprocessing cycle could easily take a century to fully burn U238 so "what it costs" is influenced by what you think interest rates will be (or should) be for next 100 years.

Fuel cycle costs are nearly zero compared to the cost of the steam turbine at an LWR. If the price of uranium tripled, it would make little different in the basebar price of electricity.

Advanced fuel cycles, today, are mostly long-term insurance against Uranium prices rising significantly.

And nope, given reprocessing, breeding, Thorium, and Uranium extraction from seawater, we're not going to run out of nuclear fuel for tens of thousands of years, if ever.

So for now, it actually is waste.

Except for the fact that people refuse to bury it.

The inconvenient truth.