The consoles justify the amount they take more because they are selling hardware at a loss to bring in users, so as a developer, you are seeing direct, tangible, and ongoing benefits to giving the manufacturers a cut. Every console cycle, there is renewed investment in the ecosystem to keep users interested.
For digital platforms, the continued investment in the platform itself is both less tangible, and I would wager less overall (though we can’t know this for Steam because we don’t have access to numbers like that). The longer Steam continues as a platform, the more true this is, unless you believe that Steam will continue to improve at the same rate. I don’t see my interaction with Steam being much different 5 years from now as it is today, so it is less obvious to me that such at steep rate is justified.
Like, imagine they “perfected” Steam. They made all the features users could ever want, and there becomes no reason to make any more changes. Should they keep charging the same rate? Or, maybe a better way to frame it, would be that rather than investing some of that 30% rate into improving the platform, they invest in developers themselves to make better products, because it’s the only place left to make the platform better than it was before. This would be equivalent to just lowering the rate across the board, in my opinion.