Even in medicine, you have many different types of degrees and certifications of varying depth and expertise. On the low end of the spectrum, you have medical assistants that can and do get certifications in less than a year and are seemingly qualified to do things like administer medicine. On the other end, you have MDs that require nearly a decade of training and are qualified to prescribe medication and do surgery. In between, you have roles like RNs and PAs.[1]
I don't think any coding bootcamp is claiming you can learn just as much as a CS grad in 12 weeks [2], but you can learn enough to be a junior web developer building modestly complex CRUD apps. Speaking as a CS grad myself, there's a surprisingly large amount of inefficiency in traditional CS curriculum and you'd be surprised how much you can learn in a short period of time. You're not going to work on Google's search algorithm or Stripe's ACH gateway after 12 weeks no matter how efficient the curriculum is, but for a lot of employers a junior dev is more than enough.
In software, the analogy to nurses and doctors is that there are: web admins that mostly set up Wordpress or another CMS for small businesses, web developers that can build modest to advanced CRUD apps, and software engineers that can work on architectural software problems like GitHub's RPC filesystem or Heroku's app router. Again, different employers have different needs and not everyone needs to be working on architectural problems: there are plenty of jobs for people who can hack Wordpress and build Rails apps, the employers who hire bootcamp grads for those needs are usually pretty happy based on our research.
Bloc is online so it's a little easier for us to experiment with different program lengths than an in-person bootcamp, we have three different programs that are 18, 36, and 72 weeks respectively and roughly line up with the role definitions I'm proposing here. We just launched the longest program, called the Software Engineering Track, exactly because we thought there was an opportunity to build something with the pragmatism of a bootcamp but the robustness of a CS degree. But, we still think there's a place for the shorter programs even if they're not a substitute for a CS degree.
[1] http://www.thepalife.com/physician-assistant-vs-nurse-practi...
[2] I haven't heard of any coding bootcamps that are 6 weeks long, I think most of the in-person programs are closer to 12 weeks.