Yes, I agree that 30% is a lot. But let's look from another perspective: If a developer, for ease of calculation, sells a game for 30$ on Steam, he receives 20$. If he sells it on a competitive platform with 5% cut (that's 6x less than Steam) he gets 27$.
However, Steam is way bigger, and if a developer can sell the same game more times on Steam (33% more times to be exact), he breaks even.
More people to buy = more people to play = bigger player base => more people buy it. It is a poaitive feedback loop.
I am not arguing that 30% is good, all I am saying is I understand that Steam has to take a big cut to pay for the features it provides for "free" alongside the usual game content (cloud saves, community, workshop, utems, etc.).