You really have to make a distinction between who the subject of a lot of these studies are. It is mostly young fresh-out-of-school (or still in school) guys (and gals). They generally have very little life and business experience. They are full of ideas and passion. But few have had to endure difficult situations in their lives --personal or business.
If you take that profile and suggest that success as a solo founder is possible, well, you would be wrong. The road to success is actually a mine field for most. And these young entrepreneurs, for the most part, are simply not prepared to have a steady hand at the tiller in a storm. So, yeah, that the probability for success goes up exponentially for teams is no surprise. So long as everyone is pushing in the same direction reasonably well things move forward. Add to that guidance and adult supervision (I don't mean that in a derogatory sense at all) and it could and does work well.
Of course, there are lots of examples of teams that self-destructed as things got difficult. I watched a documentary on indie game developers a while ago and that seemed to be a common theme: As they started to get stuck in the mud that can be a never-ending project partners started to become enemies and things went from bad to ugly fast.
It is my firm conviction that a well-rounded, experienced solo founder does not need a co-founder in order to succeed. You still have to hire good people to work with you, which is a fact regardless of how many founders might be at the top.
Outside of the tight-knit university-centric circles of places like SV, finding suitable co-founders is hard and fraught with problems. And this is particularly true as you get older. Everyone can get excited about launching a new business. Few are willing to put it all on the line to make it go. I've experienced this first-hand with friends. When you finally say "OK, let's do it" and start to discuss the reality of what's expected from each founder things slow down very quickly. That's when "we should do this together" turns into "Well, let me check with my wife" or "I don't think I can put in the time". The co-founder pool is smaller and smaller as you get older and/or accumulate responsibilities.
I'd be willing to bet that the vast majority of businesses in the world are started and run by solo founders. I know a bunch of them. I am talking about everything, from a mechanic's shop to a software company. Partnerships are far more difficult than marriages to get right and the consequences of failure can be just as dire, if not more.
A lot of these discussions on HN tend to be software-startup-centric. There's far more to the world than web and mobile software startups. By comparison software startups are easy. That's why people can write these really amazing zen sounding introspective pieces with revelations on how they did this or that, failed and then did this or that, learned, pivoted, raised some money and eventually succeeded. Try doing that with a self-funded (from savings, loans, credit cards, second mortgages or all of the above) hardware startup, a restaurant or a dry cleaning business. The phrase "I'm all in" comes to mind. Most businesses cannot be built sipping latte's at Starbucks while listening to cool tunes. Be sure to consider context before reaching some of these conclusions.