The problem isn't big, it's only
tricky. Countries deal with orders of magnitude more of even more toxic "regular" waste from chemical industry. Not to mention the (again, orders of magnitude more of) waste coming out of powerplant chimneys, that's out of mind because it quickly gets out of sight.
It's tricky because of a combination of factors like long half-lives and unusual suitability for some of the material to be reprocessed into a weapon (both fission devices for nation states and terror devices for small groups, the latter more thanks to public perception than any actual effectiveness). This creates a lot of political obstacles, as it's easier to pass responsibility to someone else than it is to double-down and implement methods to reprocess, transmute or reuse the waste.