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

Although I agree with most of the reasons given by other respondents, my primary reason for preferring a single-column integer key is that it makes writing a user interface much, much easier.

If you are using some kind of list control to represent your data (a list, list view, combo box, etc) you can uniquely relate each entry back to its database representation through a single integer value stored with the item. Most pre-written components already allow you to attach an integer to each item and for those that don't, it's very easy to extend the component to do so.

If you're passing data between a server application and a web page, it's much easier to store the single identifying value in the the id attribute of the widget that represents the data than to have to compose and parse multi-value ids.