You’re welcome to sell Steam keys on your own website, and Valve won’t take any cut at all, while still providing the same services as a purchase through the store.
See https://partner.steamgames.com/doc/features/keys , at face value, it’s a completely reasonable policy.
But if you're getting the sale because of Steam itself, then we get the cut.
You can still debate whether 30% is fair even if Steam are providing the discoverability but still, it's a solid policy and they're putting their money where their mouth is.
Do you believe developers should be able to use something a company has spent the last 19 years investing in and building... for free or cheap? Is there a competitor in the market that would bring them as much exposure while not charging as much? The only thing I can think of are platforms like EGS, where they basically pay people to sell their games on there, so they can grow the platform.
I haven't done the math myself, but I'm guessing indie developers would be hard pressed to make the same money without using Steam as they do using Steam, even with the "fleecing" you're talking about.