Valve does regulate prices on steam. It's not an open marketplace where the developer gets to make these decisions unilaterally - it's the developer working with Valve as their publisher.
Personally, seeing a $2 game on Steam would damage it's perceived value for me, as mentioned above. I would be prejudiced against it.
Mobile games trended towards $1, because they're a lot smaller and simpler than traditional PC/console games.
If angry birds was on PC, it would not make many sales with a $60 price tag. It has a low price because it's not comparable to your typical PC game.
Sure there's a lot of competition on Steam, but I don't think we'll see a race to the bottom like we did on mobile, because
- PC gamers tend to want larger, more in-depth experiences which can't economically survive off a $1 price tag.
- PC gamers aren't as tolerant of in-game advertising, which is the mechanism through which most mobile games make their money these days. The typical successful mobile game these days is free to play, but earns millions off ads. That business model doesn't translate to PC very well.
- PC gamers are accustomed to new games costing $60.
The last one is pretty important. New release console games in Australia have always been AU$100+... When the US$ crashed, making 1AUD worth more than 1USD, did retailers drop the price of games here? No, they kept selling them at AU$120 -- double their retail price in the USA, because that's what the local market was accustomed to paying. Even on Steam, there's region specific pricing, so the price of a game can be doubled if they detect that you're using an Australian IP address / credit card...
Also, more importantly, simply dropping the price of your game from $10 to $2 is not going to get you 5x more sales, which means it's not a smart business decision. PC game prices tend to be elastic, but not perfectly so.
It's a better decision to spend money on marketing to actually increase sales. This is what AAA games do -- they spend $10M making the game, and then $10M on telling people to buy it. From that $10M marketing spend, they might generate 2M sales (which means a user acquisition price of $5/user)... At $50 each, that's $100M retail, $70M wholesale, profit of $50M after deducting costs, and $25M after tax. On the other hand, if they released it for $2 and spend nothing on marketing, they would get somewhere between zero and 2M sales, bringing in $0 to $4M retail, $0 to $2.8M wholesale, and a loss of $-10M to $-7.2M after deducting costs...