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

Your question is strongly related to the surrogate (or artificial) keys vs natural keys alternative. I think it's not that composite keys are less used, but that natural keys (be them composite or simple) are less favoured than artificial keys.

Traditional relational database theory dealt mostly with "natural" keys, (the ones which have meaning from the business-domain point of view) and in that scenario composite keys are frequently found... naturally.

But in the later years, database design has favoured (though not exclusively) the "artificial" (surrogate) key pattern, typically a sequential number that has no business meaning, only serves to uniquely identifies the record in the table (and perhaps the object in the upper layer).