Why can't we assign not null constraint as table level constraint?

Why can't we assign not null constraint as table level constraint in oracle?

Not NULL is a column level constraint to ensure that any value in that column is not null, hence can’t be used as a table level constraint. One can however use it on multiple columns as per the need. Also it can be applied on table level using the ALTER command.

Eg:
ALTER TABLE emp
MODIFY(ename CONSTRAINT EMP_ENAME_NNULL NOT NULL);
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