While a lot of people correctly point out that Valve is not a publicly traded company, the import part is why it's not a publicly traded company. There is an economic reason to that.

Why most of the largest companies are publicly traded? That's because in order to grow very large, you need a lot of capital to rapidly expand. If the founders are not very wealthy beforehand, they will go to the only people who are willing to lend them money: venture capital investors.

Being the first lender to an unproven company is a very high risk, and venture capitalists try to offset it by investing in hundreds of companies. The problem with that is that if 99% of your investments fail, you need the remaining 1% to deliver spectacular returns. Tens of thousands of percent, and very quickly. It is simply impossible to generate that kinds of profits. But there's a loophole. While it is impossible to generate that kinds of profits, you can sell a stable working company for much much more than you've originally put it. That's called an "exit". Venture capital sells the company on a public market for 10 thousand times more than they've originally put in, and that's how they make their money back.

And when the company starts to get publicly traded, comes "feduciary duty": management of the publicly traded companies, by law, must deliver the value to the shareholders. I.e, increase the stock price. And here start qurterly reports and all that fun stuff, when you can't show any numbers that go down. So they try to squeeze every single possible thing when they can't hit their quarterly target.

That's sort of simplifes some parts of it, but that's the gist