Yes, the profit is excessive, but it's because they have a good product where the competition has not really been putting in much effort and letting Valve get away with it for so long.

Valve's goal isn't to maximize profit because they don't have shareholders that demand it. If they really wanted to maximize profits then there's a whole lot more to squeeze out of Steam and the games they made. And yes I agree Valve can lower their cut and still make bucket loads of money, but I highly doubt that if they did reduce their cut it would actually lead to cheaper games except for a maybe a few. Because just like Valve, the devs and publishers are profit driven and why would they turn down a potentially bigger profit?