"I think the statistics are pretty favorable to having a co-founder"

It's all in the details. Let's say we have the same success rate for single founder and multiple founders ventures. There are many more multiple founders startups for at least two reasons: (1) it takes some guts to go alone and (2) VC/angels won't fund solo undertakings. Since the multiple founders pool is bigger, there are more successfull startups of this kind. Here you've got your misleading statistics.