I imagine it's because, at that point, you're running a real company, with needs for marketing, sales, health insurance, company policies regarding leave and al

I imagine it's because, at that point, you're running a real company, with needs for marketing, sales, health insurance, company policies regarding leave and all the other adult stuff that most people expect at a real company these days. And generally, the person who started the company doesn't have the skills/knowhow to handle all that (many were just line engineers or what have you the year or two before). These things can obviously be learned and dealt with, as evidenced by the fact that there are companies that continue after the stumble.