I probably don't understand all their problems, but it certainly looks like a lot of them come their...how do i put this? either ignorance or stupidity.
I'm not just talking about the crappy companies with low budgets. On more than one occasion, it was for companies with big budgets(either unicorns, scale-ups or startups who just raised millions, and it really was the recruiter/headhunter who was screwing it up.
One of my first strategy would be to use a model to detect the candidates who are the most likely to quit their jobs(I was about to build that but I saw a startup launched something similar) (of course I shouldn't even mention "open to work" or out of work devs, that's obvious)
2nd, I would do scoring based on exact keywords matches(some of them don't even do that correctly), then on similar technologies, ie:angular/react/vue, java/c#, on which roles they used it at, for how long etc...
3rd target the employees of companies with low/declining glassdoor ratings
4th, I may scrape github accounts, to find people who just started learning a technology but may not have it mentioned on their job experience
What I don't get is, you can only contact 100 to a few hundreds people per week on linkedin, so why they do they target us so badly? Sometimes it looks like they just look for the word developer and send a message to the 100 first results
It's not like they even use fake accounts to have unlimited messages, I've rarely seen recruiters with even just 2 accounts, so they waste the small amount of messages they have on totally awful matches
I'm certain with a decent algorithm and 10-50 linkedin accounts I could do the job of 10-50 headhunters