I think you've led a charmed life, Jan! Agencies seem to have been the main way to find IT work here in the UK for many years now, especially in the freelance market. Employers still recruit directly e.g. advertising via their own websites, and you can always send in a speculative application or contact people informally, and the common wisdom is that many jobs are never advertised, so this kind of approach may be the only way to access those jobs. It's often hard even to find out who you would contact at a particular company if you wanted to send a speculative application. Some companies allow you to do this via their websites, but I don't think anybody ever reads these. But many companies have now outsourced a lot of their recruitment to agencies, so you may just be told to contact the agency anyway. Almost nobody advertises IT jobs in the printed press any more, and agencies usually advertise on the big job sites online, so it's often easier for employers to reach a larger market of potential applicants this way.
I guess the employers' view is that they can save themselves a lot of hassle this way, especially when many ads attract hundreds of applications from that large market of candidates. But as you say, agencies don't usually understand much about the technical aspects of the role, so they tend to use a "buzzword bingo" approach to filter out applications, which doesn't really help to identify good candidates, just those with the right keywords on their CVs. This means that it gets harder for experienced IT workers to find a job if they don't have exactly the right mix of skills and software versions, even if they could pick up the "required" skills in a couple of weeks on the job, while employers complain they can't find the skilled IT staff they claim to need.
Personally, I think poor recruitment practices by both agencies and employers have contributed to this so-called "IT skills shortage" in the UK, because recruiters try to filter the flood of applications by adding more and more "required" skills, until they overshoot and discover that nobody has 10 years of
Java 7, fluent Sanskrit and degrees in nuclear physics and fine art. Then everybody throws their hands up in horror and complains about the "skills shortage", before handing over the keys to the their annual IT budget to a fat consultancy that claims it can find all those "required" skills offshore. But that's another rant...
Meanwhile, in the freelance market, almost all jobs are advertised via agencies in the UK (including many freelance jobs in the Netherlands and elsewhere in Europe as well), partly because it's often easier for both parties to let the agency act as an intermediary for the contract, especially when the job might only be for a couple of months. You can find contract work directly as well, but it's usually via former colleagues/clients, and as a freelancer you have to feel confident that the client will pay you, so the agency may be a necessary evil.
Either way, if you're looking for a job, you have to look where the jobs are, not where you'd like them to be. You may prefer shopping in a colourful local market, but if Aldi or Walmart is the only option, it's better to shop there than starve!