If you have to hire the senior engineers anyway, why not hire only the senior engineers, at least in the early stages of the company? They'll be able to get actual work done, you'll have a decent codebase, and you might actually be able to ship a product that can get you additional funding and revenue. Then, once you've taken care of the money problem and the startup is no longer fighting for its life, you can hire some junior devs to flesh out the product and exploit market opportunities that you didn't have the resources to explore in the startup phase.
Over longer timescales (years), this relationship doesn't hold true. This is why Google, Microsoft, and many of the other big players in the industry can afford to hire junior engineers. That one senior engineer and five junior engineers will turn into six senior engineers, all of whom have worked closely together, and then you have a gelled team that can accomplish great things.
But you have to get there first. When a startup's fighting for its existence daily, it's not cost effective or prudent to make an investment that will take a couple years to pay off, when there's a good chance that your company will be out of business in six months.
You jump from that stage to Google as if there's nothing in between, but almost every other company is in between. Plenty of startups have been around for years, and plenty of them have enough funding to ensure that they'll still be around for years. At that point it becomes possible to make the short-term sacrifices necessary to ensure long-term success, like training.
It worked, well, terribly. The startup folded in a year and a half mostly because of code-quality issues. I remember talking to the founders near the end of my gap year, where they said that of the 10-ish or so high schoolers they hired, only two of us really produced more than we cost (even on our piddly $20-30k/year salaries), and even then we needed a fair bit of management support.
These were also not random people - one of the founders had been a teacher at my high school (he'd been an entrepreneur with a successful exit before then), and he basically skimmed the top of the class off and got them to work for him. Many of us later had successful careers at Google, Bain, or hedge funds.
But there's only so much that you can do to compensate for inexperience. There's a lot that you learn in your first job or two, even if you already know how to program. And until you learn it, there's a good chance that you'll be negatively productive.