You are correct that dishwashers are designed to be relatively sealed, which limits the escape of steam during the washing and drying cycles. The drying time or cycle in a dishwasher serves a specific purpose despite this sealed design for a few reasons:
Condensation Drying: Many dishwashers use a method called "condensation drying" to dry the dishes. During the final rinse cycle, the dishwasher's heating element heats up the interior air and water. When the cycle finishes, the hot air inside the dishwasher comes into contact with the cooler walls and dishes. The steam in the hot air condenses into water droplets, which then drip down and are drained away. This process effectively dries the dishes.
Heated Drying: Some dishwashers have a heating element dedicated to drying. When the drying cycle begins, this element heats up, raising the temperature inside the dishwasher. As the heated air circulates, it accelerates the evaporation of water from the dishes, which then condenses on the cooler surfaces and drains away.
Air Circulation: Despite being relatively sealed, modern dishwashers have a vent system or a fan that helps with air circulation. This system allows some moisture and steam to escape, helping to dry the dishes more efficiently.
Drying Aids: Some dishwasher detergents include drying aids or rinse agents, which reduce the surface tension of water, helping it to spread out and evaporate more quickly. This, in turn, enhances the drying process.
While a dishwasher may not achieve the same level of dryness as hand-drying, it is generally efficient enough to make the dishes dry and ready to use. The combination of condensation drying, heated drying, air circulation, and drying aids ensures that most of the moisture is removed from the dishes during the drying cycle, even though the dishwasher is hermetically sealed to a large extent.