It's really just a special case of the general spam problem. It costs recruiters very little to send out their listing to as many people as possible. People who are unqualified or uninterested in the specific job, or not looking, will typically just not respond. All the recruiters need to do is filter the dunning-krueger responses, pass along a hoard of largely self-filtered candidates to the hiring manager, and try to collect a paycheck.

The only real solution is to somehow make the cost of contact more expensive. I'm not sure of any way to do that other than simply trying to convince enough people to waste enough recruiter time collectively to make the dragnet approach unprofitable. And to be honest, given the churn in the industry, it might be unprofitable anyway- but there are always hoards of new graduates ever year without any particular marketable skills looking to leech onto the industry who become recruiters only to burn out after a few years anyway.