Because biological growth increases the number of cells, and more cells need more water to survive than less cells.

Interesting thought, but in this case, simplistically put, the water, a limited resource, is the food for the cells. The population will grow until no more food is present and will reach balance in the eco-system. And the cells that die recycle back into the system

And then the sealed garden would have a lot of dry tissue that for some reason can't be seen in the photo, or traces of fungus decay. The other option is that somebody is cleaning it even if does not water it, and in the process is inadvertently watering it

If the garden is opened sometimes or the cork is loose, new water will enter by gass difussion until reaching an equilibrium. So a garden is either fully sealed, or watered.

Many plants are able to drink 'mist'. Some even only drink mist in their entire life.

Yes, and then what happens when they don't get more water?

In this species in particular, first the leaves die, then stems shrink until a fraction of their former diameter, then split in segments and collapse. Each segment emits roots and leaves later.