Because it’s reasonably cheap to buy two decades of fuel in advance, and it won’t take up much space. If such a thing were a concern, then people would do precisely that.

If it were still a problem towards the end of that period, then you can in fact extract uranium from seawater.

This is true even before you start talking about reprocessing spent fuel with breeder reactors.

It's also plentiful in many countries, it's just not mined because it's too cheap. But since fuel is under 1% of the cost of Nuclear, if it was a problem we could easy mine more difficult places.

Or start recycling the "waste". Which still contains >95% of the energy of the original fuel.

Problem is: raw Uranium is too plentiful (=cheap) for recycling to be worth it.

"Problem"