Cover letter writing is often not taken into consideration while submitting your resume/CV. Never expect it done unless you are sure about it. As said earlier, employers do not have time to read resumes. Especially when they know that they did not require you to submit your resume/CV. How do they know this?
Simple! They see your cover letter. Therefore it is necessary that you make your own chances by presenting them a cover letter that is professionally written. And make them believe that it is worth seeing your resume/CV. Seeing? Yes! employers never give more that 45 sec to see a resume. But the point is that it is your writing capabilities which will impress your prospective employer.