because filtration is cheaper for solids.
The greater problem is dissolved salts, but these can be (and are) turned into solids by chemical means.
Its all cost though.
If the oceans were man made no doubt you would call for all the 'toxic uranium and thorium, that represents a risk to man for billions of years' to be removed and would then complain about the '10 trillion tonnes of waste that no one knows what to do with'...
"Uranium's average concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million,[8][13] or about 40 times as abundant as silver.[10] The Earth's crust from the surface to 25 km (15 mi) down is calculated to contain 10^17 kg (2×10^17 lb) of uranium while the oceans may contain 10^13 kg (2×10^13 lb).[8] The concentration of uranium in soil ranges from 0.7 to 11 parts per million (up to 15 parts per million in farmland soil due to use of phosphate fertilizers), and its concentration in sea water is 3 parts per billion.[13]"
So the total amount of 'toxic nuclear waste' on the planet is AT LEAST
100 quadrillion tonnes, and of that ten trillion tonnes is already in the seawater, God not having had the sense to put it in vitreous steel containers before he buried it in the ground.,So it really makes sense to worry about 10 tonnes of plutonium (less than a cubic meter) doesn't it?
Facts are such a bore, aren't they? Now watch while you wriggle and squirm and claim that somehow a neutron off plutonium or a beta particle is somehow more dangerous than one off radon...