Silver coins can spot, tarnish and who knows what else. Buying a perfect Slab is also a small guarantee that the coin stays perfect. The premium on these can possible be recouped when selling, probably easier than on non graded coins.
If people that grade coins often, sell raw coins on here, I assume they think it will not grade perfect and would be cautious to buy it.
I collect gold pf70 coins, well aware I often buy the Slab and not the coin. But I like having the nuts, and they will be easier to sell than lesser grades.