Why do plants wilt if not watered?

You can think of a plant as a bit like a tower made up of cells instead of bricks. Plant cells are mostly water. If the plant loses too much water through evaporation (or transpiration) and this isn't replaced because you haven't watered it, then the water content of the cells starts to fall. Like partially inflated balloons, they start to lose their shape and strength, becoming "flaccid" or floppy. A tower made of floppy bricks doesn't stand up very well, and is likely to bend over or wilt. Watering the plant will allow the cells to refill and the plant may recover.