Who is even looking at cover letters at all?
The problem with cover letters is false positive (people who did not write the letter themselves, or who did it to a much higher standard than their usual). But then, that’s what interviews are for.
I did hire a couple of people with either a subpar CV (some good people sometimes end up in dead ends or difficult situations), or cover letter (not everyone is a great writer), or reference (your issues with your former boss are not always your fault), or interview (you can have a bad day), so I would not rely on a single factor. But a combination of 2 dodgy elements is an automatic rejection. Each one tends to surface different aspects.
Nobody is spending 10 minutes writing a cover letter.
Companies that are getting hundreds to thousands of applications are not reading every cover letter.
Show me who you are and I’ll show you who I am, and if we like eachother, then let’s try to get together and see what we could build together. if the entire interview process is a series of hoops that you dictate I jump through to even have a chance with you, then why should I expect my future with you to be any different? I don’t want that.
I don’t want to write a carefully crafted letter pretending that I want anything other than respect and money and a chance to do work on interesting from a potential employer. I just want to write code. The rest of it is extensive shit testing in my book and I have no patience for that. I don’t put up with that kind of treatment from anyone else - what makes you think dangling money in front of me would change my mind about that?