When I apply for a job it takes me at least an hour if not more. I adopt my resumé, write the motivation letter. If I spend less then 15 minutes it means I'm doing it as a requirement from the social benefits agency, for a job that I don't want.
Only to find out that the other side didn't read that letter, and most didn't read the resume.
I know, because I asked.
I believe people in the hiring process are looking to work efficiently and a cover letter is just more stuff to read. They want to just read your resume because that's what will get passed around and what matters. Anything in your cover letter is mostly things they want to know in the interview.