The market has spoken, no one has been able to provide a better service for less, so 30 stays. Technically they could even have increased the cut because of that if you follow market rules.

But even then, you can get a lower cut on Steam if you are a very big publisher, so the 30% figure is not set in stone.

And again, the market is not gaming as a whole but gaming platforms, of which there are very few players, and none have tried to really compete with steam as a platform. Most publishers platforms were entirely driven by maximizing their profit, not competing. Most of them are basic digital storefronts, and not platforms.