They are mining uranium because it is cheaper to mine and process it into new fuel rather than recycle or burn spent fuel in fast breading reactors...
Some say that fast breeder reactors "... could power the world indefinitely; spent uranium supplies in the U.S. alone could power the country for 100 years, he said.". https://en.wikipedia.org/wiki/Breeder_reactor#Discussion
Maybe we can extract uranium from seawater in the future: https://engineering.stanford.edu/magazine/article/how-extract-uranium-seawater-nuclear-power