Because we already know how to store it for thousands of years with minimal leakage without creating a contraption that could lead to nuclear waste raining down on our heads.
To be fair, we already know how to store it with minimal leakage for thousands of years, but the tentation to just dump it in the Mediterranean (1980's), Somalia (1980-90's), seed it over Irak (Gulf's war) or leak it into the Japanese Sea (2011-17) is irresistible.