Why can’t we assign not null constraint as table level constraint in oracle? [closed]

• I have to write when create table in SQL with NOT NULL CONSTRAINT:

CREATE TABLE employees ( last_name VARCHAR2(25) CONSTRAINT emp_lname_nn NOT NULL,...);

• And I cannot write it such like:

CREATE TABLE employees ( last_name VARCHAR2(25),...,... CONSTRAINT emp_lname_nn NOT NULL (last_name));

Why Oracle put restriction especially on NOT NULL CONSTRAINT?