Water is wet because water is wet.
Grass is green because ... okay, why the fuck is grass green?
water is wet when its in liquid format. rather than solid(ice) or gas(steam) format
grass is green when it has plenty of sunlight and water. grass can be brown when it does not get satisfactory amounts of sunlight and water
saying the simple x is x because its x is simplifying things down to toddler explanation without actually explaining the how or why