I am adequate at sales but still, I hate cold calling and cold emailing - and I have weeks, sometimes months when I just can't bring myself to talk to the prospects even if it's not a cold outreach. And lastly, it's really is a full time job that needs focus and is a source of constant interruption if I'm also trying to develop some software at the same time. It doesn't go together well.
Let me just take care of the computers, that's where I shine.
One reason to want a business co-founder to pair with a technical founder is that the business side of a growing startup is a full time job, and sometimes too much for a technical founder, and/or sometimes too far outside their comfort zone. VCs also (in my experience) tend to like the CEO to have a certain mentalities about how to do business that some technical founders may lack, especially first-time technical founders. Things like willingness to pivot, views on taking responsibility, how good at pitching and sales they are in the broad sense of selling the company and the vision, and lots more stuff like that.
YC has been reticent to fund solo founders since its inception.
My conspiracy theory is at least one of the co-founders needs to be "aligned" with investors. Every startup I've worked with has had one Judas founder that became the bad guy (for employees) and steered the company, kicking and screaming, towards wherever the VCs wanted the company to go.