Because an abstract class is an incomplete class (incomplete in the sense it contains abstract methods without body and output.) We cannot create an instance or object, the same way you say for an interface.
As abstract class is a class which is defined with abstract keyword i.e.,a class which is meant not to be instantiated is by the programmer or any of its method's implementation is not known. So, what's the use of creating an instance of a class not having everything properly defined. This is the reason abstract class is not instantiated.