A couple of possible reasons. Firstly, they're a privately owned company, so unlike Plcs and corporations like Sony, Microsoft, Activision, Nintendo etc. they have no need to release sales or revenue figures.

Secondly, if their digital distribution method is successful, they don't neccessarily want everyone to know. If they start announcing huge sales figures and revenue then physical retailers may refuse to stock their games. In fact, physical retailers have already threatened stop stocking games that require Steam. The other issue is that it basically says to larger companies in the industry "digital distribution is a money maker!" and they may form their own systems and increase direct competition to Steam.

...Its because most distributors can't.

Sales data are protected by NDAs between the publisher, the platform, and the distributor. Therefore, Valve would have to break over 200 NDA's signed by the various publishers in order to give out any sort of sales data. The individual publishers/developers can, and sometimes do, but if Valve does it, it would invite lawsuits that Valve doesn't need.

There's also good chance that a big part of the profits comes from secrecy agreement that they had with the other respective companies IMHO

This is interesting. Forbes, a far more credible site, comes with a statement directly contradictin the 1 billion here.

"Steam controls half to 70% of the $4 billion market for downloaded PC games"

http://www.forbes.com/forbes/2011/0228/technology-gabe-newell-videogames-valve-online-mayhem.html

4 billion dollars?

The console market revenue is around that right IIRC? WOW.

The console market is far larger than that. Nintendo alone has a revenue of 15 billion or so (http://www.gamespot.com/news/6261400.html?tag=recent_news;title;1). Granted, that's with the DS too, but still, the console market is far larger than 4 billion dollars.

Nintendo accounts for somewhere around 45% of the total hardware sales, roughly 40% of the software is sold on their machines, and they personally publish somewhere around 15% of the software. Short and simple, they might account for around 25-30% of the revenue of the console handheld market, which would make the console handheld market revenue around 50 billion dollars.

The PC has 8 billion in revenue, though (and that's not including MMOs monthly subscriptions). Considering that there are 5 consoles/handhelds, it's bigger than the smaller ones (PsP definitely, maybe Ps3 and possibly X360/DS now).

How is Portal #6 on that list in its fourth year?

The Console software revenue is less than $20 billion, and probably closer to $15 billion. PC games revenue was estimated to be $13.1 billion in 2009, so it could be $14bln or more in 2010.

Although Nintendo's revenue was $15bln, it also accounts handheld software, handheld hardware, console hardware and accessories revenue, and probably some other stuff.