Resumes Without Cover Letters End Up at the Bottom of the Stack

When a hiring manager is seeking to hire someone, he is often bombarded with résumés. The three most common responses are résumés only, résumés with ineffective cover letters, and résumés supported with tailored cover letters. One of the first actions taken by a hiring manager involves separating the responses. More often than not, résumés without cover letters end up at the bottom of the stack. Why?