I suspect it is because most people do not have attractive options from which to select business partners...a problem that is not generally solved by founder matchmaking services. Perhaps in part because the limited options of some people are a function of their lack of attractiveness as business partners.
Guessing: because they are attracted to startups by the prospect of being free to call their own shots, and going solo maximizes that freedom... in principle, anyway. For a few lucky ones it might even work that way in practice.