Dishwashers typically (meaning I guess there may be some that don’t) have a way for the warm moist air to vent out of it. Usually in the door, and typically “hidden” on the outside to match the aesthetic.

Inside however, there is usually a more obvious way for air to escape. On my current dishwasher for instance, it’s just a removable grate at the top of the door that sits flush with the door interior itself. It’s set up in such a way that air can get through it, but not water from the cleaning cycles (since that would obviously be quite detrimental to the kitchen overall.)

Now if you think about what I just described, a small dark area with plenty of warm moist air flowing through it each time you run your dishwasher, protected from the cleaning agents and harsh spray of water, you’ll likely come to the quick realization… this is the perfect place for unwanted things to grow.