A simple answer for me: I know how hard it is.
I know I did much better as a founder when I had a true co-founder than when I didn’t.
I know that later, when you can build a true team of VPs and SVPs (say $5-$10m+ ARR), a co-founder is still important, but less critical.
But there’s just too much to own in the early days, too many cliffs to jump off without knowing how far you will fall. Having a co-pilot for me at least makes the odds of success much, much higher.
When I had a true co-founder, nothing could stop us. When I didn’t, I still did well. But I was much more mortal.