Because it's abstract and an object is concrete. An abstract class is sort of like a template, or an empty/partially empty structure, you have to extend it and build on it before you can use it.
Take for example an "Animal" abstract class. There's no such thing as a "pure" animal - there are specific types of animals. So you can instantiate Dog and Cat and Turtle, but you shouldn't be able to instantiate plain Animal - that's just a basic template. And there's certain functionality that all animals share, such as "
makeSound()", but that can't be defined on the base Animal level. So if you could create an Animal object and you would call
makeSound(), how would the object know which sound to make?