After A/B testing tons of messages (and analyzing messages from a SaaS platform I built for recruiters to send messages) the messages that work are ones that are short. So I try to keep messages brief, reach out about a specific company that might be a match, and hope to get on the phone with you to actually be able to do my actual job. It's very hard to convert someone over an e-mail message. Getting you to respond to my e-mail is the first part of the funnel.
From there if I can get a conversation with you I can move on to the "fun" part of my job. Giving candidates a broad overview of the market based on their interests, expectation setting, and career counseling.
Edit: To answer your question directly the reason you get bad recruiting messages stems from a mix of bad recruiting practices, bad companies/ low investment in HR departments, and bad engineers. Would be happy to give you a recommendation for a solid recruiter in SF or NYC if you're in those areas.