That's because building the giant plants, processing the fuel and storing the waste uses more energy than the plants produce.
However, there are those who argue that the giant plants with all those problems need not be relied on--that smaller plants using less refined and less toxic uranium as a fuel can be developed. This is an argument which seems to make sense because one of the problems with the current plants is that the uranium fuel generates much more heat than the mechanisms can contain. Which is why they have to be constantly cooled with water to dissipate the excess heat.
Also, it has been argued that there's actually an almost unlimited supply of uranium in sea water and that could, potentially be retrieved through an evaporative process, much as we retrieve salt.
As with so much else, the problem with uranium seems to arrive mainly from its concentration. Which is why I argue that what man has put together, he also need to take apart--carefully.