I think there are several beliefs that are assumed in the OP's question that may be erroneous.
First off, the assumption that the job market is effectively modeled by what's on job boards is probably faulty, especially for entry level jobs that require any specialized technical skill. In my experience entry level jobs for technical positions like programmers are often filled not through job boards, but by Networking. The problem with job boards for any entry-level position is that you get flooded with applicants and because of their limited experience, you have very little information to really discriminate among them to find just the one that you want to work with. Harring managers don't want to go through that experience, and they don't have to if they find a different way to fill the job. So they do.
Networking provides that natural filter. First of all, your network itself acts as a filter that limits the total applicant pool to only people that know somebody in your network. since you generally trust the people in your network this does a great job of filtering out a whole bunch of people that you otherwise might feel obligated to since you generally trust the people in your network this does a great job of filtering out a whole bunch of people that you otherwise might feel obligated to interview or at least read their resumes. As a result, you will see much fewer applicants in total and probably will feel better about the selection you make.
Secondly, when you are using Networking as your filter, people tend to come in at random times so often you only have to interview a single person that's a much simpler decision then to choose from tens or hundreds of candidates this is the problem of over choice.
Often, when hiring technical entry-level people the timing is not even critical. What you want to do is get to know this entry-level person and do so in a low risk way, before the need is critical.
As a hiring manager, I often recommend to people that they volunteer to do a project for someone just so that person can get to know them. Or to go to events like Hackathons, where again they are just showcasing what they can do, and their work ethic, not a resume filled with facts that have little predictive value to whether you will be a good candidate at this company or not.
Heckathorne's and other such events and programsare good for another reason, because they actually provide another way of sorting through the candidates and making a selection that is more specifically related to how you work and what you choose to do, as opposed to what you know or work that you have been assigned in the past that you might not be in love with. There may be winners.
Or there may be people who choose to work on a particular kind of app that seems most relevant to the kind of work the hiring manager might want you to do. People who are self-motivated to work in a particular area or particular technology are far easier to manage than those who are willing to do what's inside but lack internal motivation to do master it.
Another erroneous assumption in this question is that these programs meant new Programmer's from scratch and a small number of days or weeks. More likely they provide a opportunity for someone to showcase their interest in that technology or industrial area, and to gain more experience in it. Or even that they are learning a new programming language, but may have prior skills and experience in developing code for their own personal use, even though they have never had A formal job doing so. Or, they might just be showing if they are a very very quick study and learning new technologies, which in it self can't be a strong reason to hire them.
As a hiring manager for my startups, I have often use these techniques to select people who turn out to be self motivated, problem solving, overachievers, who often are the kind of employees who are most successful in startups to hire effectively.
Often, I do not even have a budget to hire someone extra, so I certainly would not advertise on a job board. But a volunteer comes along, finds me and makes me an offer to create something of value for me, that I can't currently fund and was therefore deferring. If they don't complete it, or don't turn out to fit with the team, the project is self-limiting, and we part as friends and they got more experience and a chance to show off their abilities or check out a team.
But if they are successful, their work then generates the revenue, or saves cost, sufficient to pay for that person to continue in that role. And I did not need to read a single resume or do more than one informational interview.