The materials are still full of potential energy, but it's much more expensive to reprocess them than to mine fresh uranium. It's even more expensive to reprocess them without incidentally releasing more radioactive contamination into the environment. (Several countries reprocess nuclear fuel now or did so in the past, but the facilities have always released more radioactive material into the environment than simple storage.)
It's kind of like why old and broken polyvinyl chloride pipes go to landfills instead of being burned as fuel in power plants. Even though PVC is flammable, the cost of burning PVC and capturing its carcinogenic combustion byproducts is a lot greater than burying waste PVC and burning fossil hydrocarbons.
In the far future, uranium mining costs might rise enough that it makes economic sense to reprocess old spent nuclear fuel. In the early days of the atomic age people thought that reprocessing and breeder reactors would be necessary because uranium was believed to be very rare on Earth. Vigorous exploration programs and new mining techniques proved this belief to be false by the end of the 1960s, and the situation hasn't changed since then. It's safer and cheaper to mine fresh fuel and just store the old fuel without any sort of reprocessing.
See e.g.
Bunn, Matthew G., Steve Fetter, John P. Holdren, and Bob van der Zwaan. 2003. The Economics of Reprocessing vs. Direct Disposal of Spent Nuclear Fuel (PDF):
https://dash.harvard.edu/server/api/core/bitstreams/7312037d...