Why do HR recruiters prefer CVs over Cover Letters?

As a hiring manager in marketing, I always get annoyed looks when I ask recruiters to be sure that all the candidates submit cover letters with their applications.

All the CVs are the same, just a list of obvious tasks for their roles. Just let me know where you worked and for how much time, and that's it. I can figure out what you were doing.

While, in a cover letter, the candidate can explain why he is applying, why he can be useful to the company and what he is bringing to the table. I can get a glimpse of the personality of the candidate, and I can read it in less than a minute.

So, why do most of the recruiters still prefer to focus on the CV?