The 30% it’s always been the standard though, so not just Valve. That figure comes from retail, where 30-50% is still standard practice. You could argue that retail has higher costs, therefore needs the higher cut, but when Valve created Steam, they probably went with what worked.
What I really hate about Steam and all online shops, is that you can’t resell something you purchased second hand. If I can resell my physical copy of a game or movie, I should be able to do the same with the digital version. Also the fact that they can remove access to the product you bought whenever they want. In my opinion, we need a law that specifies that what you buy is yours, and you get to do whatever you want with it, even if the manufacturer doesn’t like it.