Developers don't do cover letters

I've never written a cover letter. All my jobs (apart from one) have been through personal contacts. They only even wanted my CV to file with HR.

I have interviewed about 15 developers in the last year. None of them had cover letters.

I believe it. I know many programmers who are very passive interviewees, too. "It's his job to interview me" said one friend, a man I recommended for an opening at my then-workplace, The man was obviously far better than I am, though he had been unemployed for the previous year (in part due to his poor interviewing skills.) When he got rejected, after talking to both my friend and my boss, as far as I can tell, the boss was kinda busy and distracted, and my friend just passively answered the questions he was asked rather than using those questions as a way to jump in to some story bragging about himself like I would have. Also, my boss was suspicious that the only job on the resume for the last few years was working for me.

Now, my friend was able to finally get an employment agency to notice him, (one that did not know that my business, at the time, was probably less than a $10K/year affair) and as the man really was good, he was able to leverage that in to a full time job and has been working since.

The thing is, programmers use those sorts of sales skills a lot less often than other professions do, so many good programmers almost entirely lack those sales skills, which can be a huge problem if you get stuck in a situation where you don't have someone else nearby who knows you are good.