This might be true if you're talking about a startup run by a bunch of 20-somethings with no experience. These are certainly en vogue in Silicon Valley today, but hardly represent all startups.
> Cofounders often think they don't need management because they're all on the same team, working towards the same goals.
This is a sign of an inexperienced, naive founder. No entrepreneur worth teaming up with believes management is unnecessary.
> Startups are high pressure, and pressure makes people make bad decisions and lose their tempers.
Notwithstanding the fact that if your startup is a pressure cooker, you're doing it wrong, it's dangerous to believe that all people react the same way to pressure and stress. Pressure and stress lead some people to anger and poor decision making. Start a company with these people at your own risk.
> Deciding to start a company from scratch with the goal of building a billion dollar business takes ego.
Most successful businesses aren't started by people who are focused on building a billion-dollar company. If that's your overriding goal, clashing egos will hardly be your biggest problem.
Incidentally, while it obviously wasn't the intent, this post demonstrates some of the advantages of founding a company solo. It worked for Sam Walton, Ralph Lauren, Richard Branson, Sara Blakely, Jeff Bezos, Pierre Omidyar, Michael Dell, Ross Perot and countless others.
Investors like YC don't look on solo founders favorably and have helped create a narrative that has deterred some entrepreneurs from taking this path (and created the inane "seeking co-founder" environment), but entrepreneurs should understand that investors have motives for wanting multiple founders that are not necessarily aligned with their interests.
For seed stage investors particularly, multiple founders is a derisking mechanism. For instance, there are a lot of smart, talented folks out there, but it's still hard to find exceptional founders capable of competently wearing multiple hats. On the flip side, hiring exceptional employees is expensive. A company with just $120,000 in funding can't hire a good developer, product manager and sales or bizdev person. This creates significant risks for seed stage investors. The solution: seek companies with multiple founders who split key roles. Since founders at this stage essentially work for ramen and equity, having multiple founders is like a cheap insurance policy for resource risk.