one of the reasons valve might be setting their cut so high is to stop it from being a method of withdrawing steam wallet money into actual money
like when people cash out, they use PayPal and go through a trusted middleman. if Valve's cut was tiny, then you could cut out the middleman by selling keys on the market and then buying a smurf's mods in order to get money paid from valve to your bank account
I mean valve is also greedy as fuck but this probably is also a factor
the thing is, with a cut so low, modders will just have PayPal donate buttons instead because that's way more profitable
the reason Apple got away with 70% cuts was because it was their platform and there was no workaround, and they had a huge market share. Google didn't, so Google only took 30%. Valve is trying to be like Apple here but the thing is, they don't have a monopoly. This is the fucking modding community, there are always workarounds. Hell, with Skyrim, there's not even a way to implement DRM on paid mods, at least Google can do that.