The problem quite simply is that there are a lot of wannabe entrepreneurs out there who expect someone with technical chops to join them when they a) don't have an existing relationship, b) don't have a proven track record of execution and c) don't have anything to give besides equity that isn't worth anything. In other words, "co-founder" is little more than a title bestowed upon the person you need to build your product but can't actually pay to do the work.
It's no surprise that it's hard to find a "co-founder" of any type under these circumstances. When somebody you briefly spoke to at a meetup asks you to be a "co-founder" in exchange for 10% of a company that doesn't exist yet, you're naturally going to be skeptical. If that person adds a reasonable salary to the equation, you're far more likely to take the proposition seriously.
Additionally, I think it's worth pointing out that wannabe entrepreneurs have a lot of misconceptions about what type of technical skills they need. Instead of looking for someone who can build a basic web app (which is what 99% of them want to build), they look for a hipster developer whose resume is filled with all the buzzwords of the day. You do not need a Ruby on Rails developer with MongoDB experience to build a web application where individuals can organize and share pictures of their pet goats.