In terms of osmosis, wilting of plant is known as plasmolysis and it occurs when water diffuses through the semi permeable membrane from an area of high water concentration to an area of low concentration. In case the plant is placed in a very concentrated salt solution in which concentration of water is lesser than the salt concentration, the water from inside the plant will diffuse out as the concentration of water outside is lower than inside. The diffusion will continue till the water concentration is equal inside and outside. This results in wilting as plants lose the water. However, this only occurs in very extreme conditions which are a rarity in nature.
Plants are made up of cells and cells need several nutrients or factors in order for it to live,now,when the plant will not be watered there will be no water entering the cell and the result of dead plant cell is a is a wilted plant.
Because plants need water and sunlight to live. They are a self-providing evolved species of life. If they don't do what they've evolved to do they'll die.