Recruiter here, been doing it a long time and work at a fairly senior level in a very specialist (non-IT) niche. Lots of hate on here but that's pretty normal when you think about it. Recruiters are in the rejection business. For every one person that gets a job there are many (think hundreds if we run a Seek ad) that are going to be rejected. So most people's experience with recruiters will be negative. Also, the bar to entry is pretty low so you do get a lot of cowboys in the industry alongside the serious career professionals. A bit like financial advisors, or professional coaches.
All this advice to avoid recruiters is fine if you're worth $50K a year, but it is not good advice once you hit a certain level. If you are a white-collar professional, in many sectors once you get to a certain level in your career (let's say $150K+) you will be dealing with external recruiters, as even organisations with the most robust internal hiring teams need external help to find people in specialist niches. In many cases, the only path to the job in question is through the external recruitment firm that has been retained (paid some $$ up front) to exclusively run the search.
Re the job ads, don't assume that there is a 1:1 relationship between posted job ads on Seek, and open vacancies. Seek is an advertising medium, so is good for telling the world that there is a job available. It is also a massive time consumer for recruiters (internal or external) as it has a terrible signal-to-noise ratio and generates a large number of incoming phone calls that we have to politely respond to even when people are not suitable. For this reason we will do everything we can to avoid having to advertise on Seek. You want to already be known to a recruiter, so that when they get a job in, they call you straight away and you save them the time of having to write, post and respond to a job ad. With our firm, we probably advertise 20% of the roles we fill.
(Yes there are lots of dodgy recruitment agencies, and yes there are sometimes ads put up that don't correspond to real jobs in order to fill a database, which I agree is unethical. There are also a lot of other reasons to make job ads purposefully vague that aren't so nefarious.)
In regards to your question about how you can make yourself more attractive to recruiters, my suggestion is that you identify a few good recruiters in your area of expertise. They may have job ads up that are the sort of job you'd be good for, and/or they might have a LinkedIn profile that states that they place people like you.
Look for a recruiter who has been working in your specific sector for more than two or three years (that will rule out a lot), and who has been with their present firm for more than twelve months (that will rule out a lot more). Keep an eye out also for those who are qualified in the area that they're recruiting in (we do exist!). When you find them, call them, introduce yourself (keep it brief), and confirm with them that you are the sort of person that they place. During this 3-5 minute call, ask if you can send them your CV, and offer to meet with them in future as appropriate.
This call is in fact a mini-interview. Demonstrate to the recruiter that you speak English, aren't an idiot, and that you are willing to work with him/her, and you have a good chance of being put into the 'call first' pile when a job next comes down the pipe that you'd be suitable for.
If you are a new grad, are trying to change careers, or your job history isn't very good/stable, or you have terrible communications skills, a recruiter is unlikely to be able to help you.