Two people already form a 'company' and the hard part of convincing somebody else of your idea has already been accomplished. You can argue growing from 1 person to 2 is the most critical part of company growth. With 2+ cofounders this critical part is already figured out making it much less risky proposition than a single founder who hasn't convinced anybody else to join them yet.

I hate everything related to marketing, especially and very much the modern form of influencer marketing that's all about interacting with people on social media, posting on profiles, posting comments, replying to them, sending messages, etc. It feels so fake it physically hurts. Worse yet if it includes a blogspam with "a list of top 3 tools in the sector, of course ours is the best" - such a waste of internet bandwidth. Every time I see a guy doing that on LinkedIn promoting something I get a gagging reaction and angrily close the page. If it involves an Instagram profile I run away as fast as I can, I just can't stand the people there. My Facebook profile had last update in 2014 when I uploaded a bunch of photos from a vacation with friends and no further comments, and that's just about enough for me.

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.

I wouldn’t say there’s a single rationale; it depends on the founders and depends on the VCs. That said, the inherent risk of conflict between founders is actually a reason to want 2 founders, because you have some of the same conflicts with 1 founder and sometimes it can’t be resolved.

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.

The reasoning provided by YC is in the subsequent tweet: "It's pretty rare for YC to fund solo founders, because startups take many years to get big and it's very hard to stay motivated for that many years without a cofounder helping."

YC has been reticent to fund solo founders since its inception.

Contingency planning/bus factor.

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.