Resin does not dry, it cures. However, when it is in contact with air, it will remain sticky. When it is not in contact with air, such as *inside* a crack or underneath a cure tab, it will dry to a non-sticky soft plastic.
To see the difference, put two drops on a windshield and cover one with a cure tab. Cure both with a UV lamp or sunlight. After curing remove the tab and scrape both drops off with a razor. The uncovered drop will be sticky. The covered drop will be dry.
This is why the resin works (while still remaining somewhat flexible) inside the glass.