why can t sodium be prepared through hydrolysis of saltwater?

Mostly because saltwater is already hydrolyzed.
But even if you meant "electrolysis", any sodium that got formed would react instantaneously with the water to form sodium hydroxide in solution and hydrogen gas. The point being that if you try electrolysis of saltwater you get hydrogen, not sodium. [From a theoretical point of view, all this is a consequence of the relative positions of sodium and hydrogen in the activity series.]

If sodium could be formed in hydrolysis of saltwater, it reacted vigorously with water to form sodium ions again. Therefore, sodium cannot be prepared in such hydrolysis.