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

Well, it's basically about keeping JOINs simple - which one is simpler to understand:

SELECT
   p.ID, p.Name, p.City,
   c.ID, c.Country, c.ISOCode
FROM
   dbo.Parent p
INNER JOIN
   dbo.Child c on c.ParentID = p.ID

or

SELECT
   p.ID, p.Name, p.City,
   c.ID, c.Country, c.ISOCode
FROM
   dbo.Parent p
INNER JOIN
   dbo.Child c ON c.ParentName = p.Name
     AND c.ParentCity = p.City
     AND c.ParentCountry = p.Country

If you have composite primary keys, anyone joining to your table from a child table must "drag along" all those columns, and all those columns are also going to be present in the child table and the JOIN statements are pretty messy. Much better to have a single (even surrogate) key for the JOIN!