Why single primary key is better than composite keys?

Question

Why is the rejection of composite keys in favor of all tables using a single primary key named id? Cause generally all ORM follow this.

EDIT

Answer 1

The only real limitation that I have run into using composite keys regards using an IN expression with a subquery. This is a problem, because a subquery in an IN expression must return a single column (at least in T-SQL).

SELECT
    emp.Name,
    emp.UserDomain,
    emp.UserID
FROM
    employee emp
WHERE
    ???? IN (SELECT e.UserDomain, e.UserID FROM ... /* some complex 
                                                       non-correlated subquery 
                                                       or CTE */
            )

There are always work-arounds, of course, but sometimes it could be an annoyance.

This is hardly a reason to avoid a composite key in places where it makes sense to use one.