Compared to abandoned/forgotten mines (that eventually cave in) and mega-scale chemical waste dumps/sites/spills, nuclear waste sites - especially ones that'll still be seriously dangerous centuries or millennia from now - are profoundly rare.
And the tech to detect that you're digging into radioactive stuff is far simpler than the tech to detect that you're digging into some sort of chemical waste, or a failing old mine or tunnel.
If millennia-in-the-future humans care all that much about what we did with our nuclear waste, it'll either be political/ideological, or (as PaulHoule suggested) just one more "they didn't leave it somewhere really convenient for us" deal.
And the tech to detect that you're digging into radioactive stuff is far simpler than the tech to detect that you're digging into some sort of chemical waste, or a failing old mine or tunnel.
If millennia-in-the-future humans care all that much about what we did with our nuclear waste, it'll either be political/ideological, or (as PaulHoule suggested) just one more "they didn't leave it somewhere really convenient for us" deal.