I couldn't agree more. I have been a Cofounder (with 4 others) once and a single founder twice (all 3 companies are profitable and alive). Based on my experience, I feel convinced that "unless" you start together as cofounders, its just very difficult/rare to "find" a cofounder who works out. There are a variety of reasons and I have seen some in my attempts at looking for one.
Being a single founder is tough, but for those who can do it , it can work really well. There are some strategic downsides to it like decision making is based on just the single founder's gut/knowledge/experience, and its important that single founders understand the downsides and work around them.